diff --git a/src/MPSegment.hpp b/src/MPSegment.hpp
index 3ae78b9..0746907 100644
--- a/src/MPSegment.hpp
+++ b/src/MPSegment.hpp
@@ -96,6 +96,7 @@ namespace CppJieba
                 }
                 vector<SegmentChar> SegmentChars;
                 //calc DAG
+                SegmentChars.reserve(end - begin);
                 if(!_calcDAG(begin, end, SegmentChars))
                 {
                     LogError("_calcDAG failed.");