One way to meet the needs of materials is to mix two or more polymer materials, which is called as polymer blends. It is known that this largely changes mechanical properties depending on molding conditions, compatibility, form and the like. The injection molding temperature dependences of mechanical properties of PS/SEBS polymer blends with different styrene content were investigated. The mechanical properties were evaluated using a 3-point bending test, and the morphology observation was evaluated using a digital microscope and AFM. The negative correlation obtained between styrene contents of SEBS and injection molding temperature, and flexural yield strain was considered to be due to the increase of Poisson's ratio. The yield conditions of PS/SEBS polymer blends were craze which is a local shear yielding phenomenon regardless of the compatibility between PS and SEBS. The difference between the experimental value and the theoretical value of yield stress was considered to thermal residual strain. It was found that the injection molding temperature dependence on the mechanical properties of PS/SEBS polymer blends needs to take into account both Poisson’s ratio and residual strain.