This paper describes the test results of six reinforced concrete slabs with boundary frames and discusses the ultimate strength of reinforced concrete slabs. Slabs were 1.4m square, 10 cm thick and effective depth of 7.0cm. They were subjected to a centrally located, concentrated load and were supported at four corners. Variables which were taken into account in this series of test were (1) rigidity of boundary frame and (2) ratio of reinforcement.
　From the shapes of cracking lines observed, the formula of calculation of ultimate fleuxural strength based on the yield line theory was suggested. Shear failure of slab tested was considered to be shear diagonal tension failure. It was found that ultimate strength due to shear failure depended on the dowel action of reinforcement at fracture. It was shown that there was no relationship between ultimate shear strength and raio of reinforcement in range of 1.13～2.30 percent.