Abstract: This paper is based on the experiment results observed from 15 different section size beams subjected to the monotonic loading and the low reversed cyclic loading.In order to study the effect both on the section size and the variation laws for the plastic hinge length,four approaches were used to explore the variation laws when the load surpassed the yield point of the reinforcement to the limit displacement point.The analysis results show that after the yield load,the more the increase of the load,the more the increase of the plastic hinge length.It means that the plastic hinge length shows the size effect obviously.When section heights increase from 200 mm to 1 000 mm,the relative plastic hinge length of the specimens subjected to the monotonic loading shows that the peak load point and the limit displacement point are reduced by 40%and by 44%,respectively,and by 38%and by 40%for the low reversed cyclic loading.The results calculated from the proposed approaches which are considered to be the influential factor of the sectional height,agree well with the experimental results.