Size　effect　is　concrete　inherent　characteristic　and　has　an　effect　on　beam　bending　failure　mechanism.　Bending　experiments　based　on　different　section　size　RC　free　beams　were　carried　out　and　test　data　was　collected　in　different　loading　stage,　including　bearing　capacity,　deflection,　section　strain　etc.　Size　effect　factors　and　failure　mechanism　were　analyzed　according　to　different　loading　stage.　Concrete　compressive　properties　show　negative　size　effect　in　bending　member　mechanical　properties,　but　it　is　not　significant　in　overall　mechanical　behaviors.　Internal　force　arm　and　longitudinal　reinforcement　have　positive　effect　to　flexural　bearing　capacity　and　ductility.　Flexural　bearing　capacity　and　ductility　show　a　growing　trend　with　member　size　increasing.　Failure　characteristics　and　measured　data　show　significant　difference　in　specimen　size.　Related　calculation　parameters　which　have　a　relationship　with　section　height　are　derived,　and　then　ultimate　bearing　capacity　calculation　equations　with　size　effect　related　are　established.　Bearing　capacity　calculation　theories　in　current　specifications　are　proved　to　be　safe　in　research　scale.