A simplified approach for calculating the stress field of the fiber-reinforced composites is developed to improve the work of Theocaris et al. (1985) in this paper. This approach considers that the main factor affecting the stress field of composites is the existence of interphase between fiber and matrix, which possesses different Young’s modulus than those of the constituent phase. A parabolic law is adopted for the variation of Young’s modulus of graded interphase, versus the polar radius from fiber to matrix, while the Poisson’s ratio of the interphase is assumed as constant. Space axisymmetric model is applied to the Representative Volume Element (RVE) of the fiber-reinforced composites to evaluate stress field of the composites. Also, the effective longitudinal modulus of the fiber-reinforced composite can be obtained.