diff --git a/cpp/CHANGELOG.md b/cpp/CHANGELOG.md
index 967ba4b8b8f4da411bab0522445a92b6ddf62f10..d19723fede468bc2cc052ac722705c68ec1785e0 100644
--- a/cpp/CHANGELOG.md
+++ b/cpp/CHANGELOG.md
@@ -14,6 +14,7 @@ Please mark all change in change log and use the ticket from JIRA.
 - MS-622 - Delete vectors should be failed if date range is invalid
 - MS-620 - Get table row counts display wrong error code
 - MS-637 - out of memory when load too many tasks
+- MS-640 - Cache object size calculate incorrect
 
 ## Improvement
 - MS-552 - Add and change the easylogging library
diff --git a/cpp/src/wrapper/VecIndex.cpp b/cpp/src/wrapper/VecIndex.cpp
index 6bc054bd21cbceee9963ee2404dee51015e1ffaf..abf97e69e59339d7e5b9fe8c788400c20044ee5b 100644
--- a/cpp/src/wrapper/VecIndex.cpp
+++ b/cpp/src/wrapper/VecIndex.cpp
@@ -37,9 +37,17 @@ namespace engine {
 
 int64_t
 VecIndex::Size() {
+    if (size_ != 0) {
+        return size_;
+    }
     return Count() * Dimension() * sizeof(float);
 }
 
+void
+VecIndex::set_size(int64_t size) {
+    size_ = size;
+}
+
 struct FileIOReader {
     std::fstream fs;
     std::string name;
@@ -159,12 +167,13 @@ GetVecIndexFactory(const IndexType& type, const Config& cfg) {
 }
 
 VecIndexPtr
-LoadVecIndex(const IndexType& index_type, const knowhere::BinarySet& index_binary) {
+LoadVecIndex(const IndexType& index_type, const knowhere::BinarySet& index_binary, int64_t size) {
     auto index = GetVecIndexFactory(index_type);
     if (index == nullptr)
         return nullptr;
     // else
     index->Load(index_binary);
+    index->set_size(size);
     return index;
 }
 
@@ -210,7 +219,7 @@ read_index(const std::string& location) {
         delete[] meta;
     }
 
-    return LoadVecIndex(current_type, load_data_list);
+    return LoadVecIndex(current_type, load_data_list, length);
 }
 
 Status
diff --git a/cpp/src/wrapper/VecIndex.h b/cpp/src/wrapper/VecIndex.h
index 34d3d2f761c01b262ea0f24ac41c9b10265bb2ef..f5fdd49466bcd4ce5c8f2ff484dbe8103503f5bd 100644
--- a/cpp/src/wrapper/VecIndex.h
+++ b/cpp/src/wrapper/VecIndex.h
@@ -87,6 +87,9 @@ class VecIndex : public cache::DataObj {
     int64_t
     Size() override;
 
+    void
+    set_size(int64_t size);
+
     virtual knowhere::BinarySet
     Serialize() = 0;
 
@@ -115,6 +118,8 @@ class VecIndex : public cache::DataObj {
         return Status::OK();
     }
     ////////////////
+ private:
+    int64_t size_ = 0;
 };
 
 extern Status
@@ -127,7 +132,7 @@ extern VecIndexPtr
 GetVecIndexFactory(const IndexType& type, const Config& cfg = Config());
 
 extern VecIndexPtr
-LoadVecIndex(const IndexType& index_type, const knowhere::BinarySet& index_binary);
+LoadVecIndex(const IndexType& index_type, const knowhere::BinarySet& index_binary, int64_t size);
 
 extern IndexType
 ConvertToCpuIndexType(const IndexType& type);