This　paper　carries　out　experimental　research　on　size　effect　of　five　different　section　size　free　beams.　The　total　number　of　reinforced　concrete　specimens　is　13.　According　to　observing　failure　process　in　different　phases　and　analyzing　test　date　including　section　bending　moment,　deflection　and　sectional　strain　etc.　In　research　scale,　evaluating　size　effect　on　mechanical　properties　of　concrete　materials　under　compression　zone　in　beams.　Characteristic　parameters　are　measured　and　derived　with　size　changing,　and　safety　of　calculation　theory　about　ultimate　strength　of　flexural　members　is　basically　verified.　It　is　also　found　that　code　value　of　ultimate　strain　under　non-uniform　compression　is　bigger　than　test　value　in　large-size　flexural　members.　Ultimate　deformation　capacity　of　concrete　materials　under　compression　zone　is　becoming　smaller　with　specimen　size　increasing.　Due　to　failure　characteristics　in　the　state　of　ultimate　bearing　capacity　have　a　closely　link　with　it,　this　should　been　paid　much　attention.