Abstract: The law of tensile strength of short fiber/rubber composites(SFRCs) was studied for the first time among a wide range of fiber volume and length. Based on the Cox shear lag and characters of SFRC, a new mixture law was put forward and the longitudinal tensile strengths of SFRCs were theoretically predicted.The results show that the new model includes more comprehensive structure factors and predicted values can well suit experimental values. In addition, the mechanism of tensile fracture of SFRCs was investigated; it was thought that fracture of SFRCs was controlled by the interface between fibers and matrix and there existed a competitive course between the increase of interfacial shear stress and increase of tensile stress on the fibers, where the former restricted the latter.