Many　researches　on　size　effect　of　quasi-brittle　materials　have　been　carried　out.　But　experiment　combining　with　RC　structures　is　not　sufficient.　At　present,　design　values　in　rules　and　regulations　are　mainly　based　on　experimental　results　of　small　size　test　samples,　which　exist　different　from　practical　engineering　structures.　Meeting　structural　normal　using　function　is　a　basic　principle　of　structural　design.　In　the　process　of　designing　and　checking　to　deformation,　calculating　short-term　stiffness　at　using　stage　is　the　most　important.　This　paper　based　on　observing　experimental　phenomenon　and　analyzing　test　data,　evaluating　size　effect　on　short-term　rigidity　at　using　stage　of　RC　flexural　members.　Obtaining　size　modify　relation　to　correlation　calculation　formula.　This　research　provided　reference　for　practical　engineering　design.　©　2011　IEEE.