The tensile model of foams is built using UG software and the tensile deformation processes of microcellular polypropylene foams with different cell sizes are simulated using finite element method (FEM). The effect of cell size on mechanical property is evaluated based on the microstructure of the foams. The cells with small and uniform size are in a state of plane stress, which improved effectively mechanical property of the foams. Whereas the cells with large and nonuniform size are in a state of plane strain, which leads to low mechanical property. The simulation results coincide well with experimental results.