Experimental　studies　on　size　effect　of　flexural　behavior　of　reinforced　concrete　beams　are　not　carried　out　fully.　Standard　values　are　based　on　test　results　of　small-size　components,　which　are　different　from　large-size　components　in　practical　engineering.　For　size　effect　on　quasi-brittle　materials　such　as　concrete,　related　researches　have　been　carried　out　for　many　years,　but　related　test　study　combined　with　concrete　structures　or　components　are　not　sufficient.　This　article　is　based　on　experimental　study　of　13　reinforced　concrete　free　beams,　obtaining　test　data　during　different　loading　stage,　such　as　carrying　capacity,　deflection,　steel　and　concrete　strain　etc.　Test　results　show　that　size　effect　of　flexural　behavior　of　RC　free　beams　is　mainly　reflected　in　reinforcement　yielding　stage　and　concrete　crushing　stage.　The　negative　size　effect　due　to　concrete　material　is　not　significant.　Yet　internal　force　arm　and　longitudinal　reinforcement　have　a　positive　size　effect　on　flexural　behavior,　strength　and　ductility　reserves　show　a　growing　trend　with　specimen　size　increasing.　The　safety　of　calculation　theory　of　bearing　capacity　in　code　is　verified　indirectly.