diff --git a/src/thread/AsyncBack.cpp b/src/thread/AsyncBack.cpp
index 5bdada5..70e99cd 100644
--- a/src/thread/AsyncBack.cpp
+++ b/src/thread/AsyncBack.cpp
@@ -20,9 +20,10 @@ bool AsyncBack::newTaskOK(const MT_TaskEntryFN& mt_aEntryFN, const TaskBackFN& a
     fut_backFN_S_.emplace_back(  // save future<> & aBackFN()
         async(
             launch::async,
-            [mt_aEntryFN]()  // must cp than ref, otherwise dead loop
+            [mt_aEntryFN, this]()  // must cp than ref, otherwise dead loop
             {
                 const bool ret = mt_aEntryFN();
+                this->nDoneTh_.fetch_add(1, std::memory_order_relaxed);  // fastest +1
                 mt_pingMainTH();
                 return ret;
             }
diff --git a/src/thread/ThPoolBack.cpp b/src/thread/ThPoolBack.cpp
index a53fc30..dba265c 100644
--- a/src/thread/ThPoolBack.cpp
+++ b/src/thread/ThPoolBack.cpp
@@ -41,6 +41,7 @@ ThPoolBack::ThPoolBack(size_t aMaxThread)
                     this->taskQ_.pop_front();
                 }
                 task();
+                this->nDoneTh_.fetch_add(1, std::memory_order_relaxed);  // fastest +1
                 mt_pingMainTH();  // notify mainTH 1 task done
             }
         });
diff --git a/src/thread/ThPoolBack.hpp b/src/thread/ThPoolBack.hpp
index 506c3bf..867588d 100644
--- a/src/thread/ThPoolBack.hpp
+++ b/src/thread/ThPoolBack.hpp
@@ -11,7 +11,8 @@
 //
 // MT safe: NO (can be used in main thread only)
 // Exception-safe: NO
-// Use-safe: yes
+// Use-safe: yes with condition:
+// - shall not too many tasks that exceeds fut_backFN_S_/nDoneTh_/.. (impossible in most/normal cases)
 // ***********************************************************************************************
 #pragma once
 
diff --git a/src/thread/ThreadBack.cpp b/src/thread/ThreadBack.cpp
index 843df66..1808fb4 100644
--- a/src/thread/ThreadBack.cpp
+++ b/src/thread/ThreadBack.cpp
@@ -17,24 +17,25 @@ namespace rlib
 // ***********************************************************************************************
 size_t ThreadBack::hdlFinishedTasks(UniLog& oneLog)
 {
-    size_t nHandled = 0;
-    for (auto&& fut_backFN = fut_backFN_S_.begin(); fut_backFN != fut_backFN_S_.end();)  // may limit# if too many
+    size_t nHandledTh = 0;
+    const auto nDoneTh = nDoneTh_.exchange(0, memory_order_relaxed);
+    for (auto&& fut_backFN = fut_backFN_S_.begin(); nHandledTh < nDoneTh && fut_backFN != fut_backFN_S_.end();)
     {
         // - async() failure will throw exception -> terminate since compiling forbid exception
         // - valid async()'s future never invalid
         // - valid packaged_task's get_future() never invalid
         auto&& fut = fut_backFN->first;
-        // HID("(ThreadBack) nHandled=" << nHandled << ", valid=" << fut.valid() << ", backFn=" << &(fut_backFN->second));
+        // HID("(ThreadBack) nHandledTh=" << nHandledTh << ", valid=" << fut.valid() << ", backFn=" << &(fut_backFN->second));
         if (fut.wait_for(0s) == future_status::ready)
         {
             fut_backFN->second(fut.get());  // callback
             fut_backFN = fut_backFN_S_.erase(fut_backFN);
-            ++nHandled;
+            ++nHandledTh;
         }
         else
             ++fut_backFN;
     }  // 1 loop, simple & safe
-    return nHandled;
+    return nHandledTh;
 }
 
 // ***********************************************************************************************
diff --git a/src/thread/ThreadBack.hpp b/src/thread/ThreadBack.hpp
index c09736b..d55ef93 100644
--- a/src/thread/ThreadBack.hpp
+++ b/src/thread/ThreadBack.hpp
@@ -10,6 +10,7 @@
 // ***********************************************************************************************
 #pragma once
 
+#include <atomic>
 #include <functional>
 #include <future>
 #include <list>
@@ -54,7 +55,8 @@ class ThreadBack
 
 protected:
     // -------------------------------------------------------------------------------------------
-    StoreThreadBack fut_backFN_S_;
+    StoreThreadBack      fut_backFN_S_;  // must save future till thread end
+    std::atomic<size_t>  nDoneTh_ = 0;   // improve main thread to search done thread(s)
 };
 
 }  // namespace
@@ -62,4 +64,5 @@ class ThreadBack
 // YYYY-MM-DD  Who       v)Modification Description
 // ..........  .........   .......................................................................
 // 2024-07-09  CSZ       1)create
+// 2024-08-05  CSZ       - nDoneTh_ to improve iteration of fut_backFN_S_
 // ***********************************************************************************************