In this paper two numerical methods for analyzing a composite beam are proposed, which are based on the hybrid stress model. One assumes changeable effective width of concrete slab. The other assumes two dimensional stress distribution in a concrete slab. Both considers interface slip between top flange of a steel beam and a concrete slab. And these numerical methods are applied to predict the plastic behavior of two composite beams (CB 5-D, CB 5-DV) whose experimental results are reported by Dr. Igarashi et. al. (13)