Using a new quasi-sphere model, an analytical method is presented to determine the material properties and stress concentration factors of rubber-toughened polymers. The calculation is simple and the closed form results fit quite well with experimental data and various results using the finite element method. This simple and efficient method can be easily used to calculate a huge amount of results to fit the requirement of engineering. Also, this type of modeling can provide a better understanding of deformation mechanisms for development of this important composite material.