In this chapter, the critical stress transfer factors of interface material have been studied under tensile loading. The polypropylene (PP) short fiber was embedded into the polypropylene co-ethylene (PPE) cylindrical interface first and then into the matrix material. Modified interface PPE material with lower elastic constant value than matrix material was used in our study. In this chapter, interface parameters affecting the stress transfer mechanism have been investigated. Finite element analysis (FEA) package (Ansys) has been used in the numerical modeling by using representative volume element (RVE). Tensile load was applied on one side of the composite cylinder as the other side of the composite is fixed. The critical stress-strain distributions are determined and presented by curves and tables for different fiber and interface diameters. For verification, the equivalent elastic material constants have been compared with the analytical solution and the results have been appropriate.