diff --git a/ecmascript/base/json_stringifier.h b/ecmascript/base/json_stringifier.h
index ec71dc6976f2456a6be47d1768ac35950d7489fc..b99f6092f97f4a554a2afed08a133979f849c977 100644
--- a/ecmascript/base/json_stringifier.h
+++ b/ecmascript/base/json_stringifier.h
@@ -36,6 +36,7 @@ public:
     {
         encoding_ = Encoding::ONE_BYTE_ENCODING;
         indent_ = 0;
+        oneByteResult_.reserve(INIT_RESULT_LENGTH);
     }
 #else
     explicit JsonStringifier(JSThread *thread, TransformType transformType = TransformType::NORMAL)
@@ -125,6 +126,9 @@ private:
         ONE_BYTE_ENCODING,
         TWO_BYTE_ENCODING
     };
+
+    static const uint32_t INIT_RESULT_LENGTH = 512;
+
     Encoding encoding_;
     CString oneByteResult_;
     C16String twoBytesResult_;
diff --git a/ecmascript/base/json_stringifier_optimized.cpp b/ecmascript/base/json_stringifier_optimized.cpp
index 8a2f42c335981cfcca2b9c8bb7fd56a4a10448c9..75e48179f0f597a901037548d934cdb447d8d210 100644
--- a/ecmascript/base/json_stringifier_optimized.cpp
+++ b/ecmascript/base/json_stringifier_optimized.cpp
@@ -1147,10 +1147,12 @@ void JsonStringifier::ChangeEncoding()
 {
     encoding_ = Encoding::TWO_BYTE_ENCODING;
     size_t length = oneByteResult_.length();
+    twoBytesResult_.reserve(oneByteResult_.capacity());
     twoBytesResult_.resize(length);
     for (size_t i = 0; i < length; ++i) {
         twoBytesResult_[i] = oneByteResult_[i];
     }
+    CString().swap(oneByteResult_);
 }
 
 void JsonStringifier::NewLine()