Six　reinforced　concrete　columns　subjected　to　monotonic　horizontal　loading　under　different　axial　compressive　ratios　were　tested　to　investigate　the　size　effect　on　flexural　behavior　of　columns.　Axial　compressive　ratio　and　section　size　of　specimen　were　different,　while　the　shear　span　ratio　and　reinforcement　ratio　remained　constant.　The　flexural　behavior　of　specimens　was　analyzed,　including　nominal　ultimate　flexural　moment,　ductility,　location　of　neutral　axis　and　length　of　plastic　hinge,　etc.　The　analysis　results　show　that　the　size　effect　of　nominal　ultimate　flexural　moment,　compressive　strain　of　concrete　at　peak　loading,　ductility,　location　of　neutral　axis　and　length　of　plastic　hinge　is　apparent,　which　decrease　as　the　specimen　size　increases;　when　axial　compressive　ratio　is　big,　nominal　ultimate　flexural　moment　and　compressive　strain　of　concrete　at　peak　loading　show　more　significant　size　effect;　when　axial　compressive　ratio　is　small,　the　size　effect　of　displacement　ductility　coefficient　is　more　apparent.　Then　on　the　basis　of　experiment,　a　practical　calculation　model　of　flexural　strength　of　reinforced　concrete　column　considering　section　size　effect　coefficient　was　proposed.　The　safety　coefficient　of　large　size　specimen　is　consistent　with　results　of　small　size　specimen.