The stress relaxation of silica (SiO2) filled solution-polymerized styrene-butadiene rubber (SSBR) has been investigated at shear strains located in the nonlinear viscoelastic regions. When the characteristic separability times are exceeded, the nonlinear shear relaxation modulus can be factorized into separate strain- and time-dependent functions. Moreover, the shear strain dependence of the damping function becomes strong with an increase in the SiO2 volume fraction. On the other hand, a strain amplification factor related to nondeformable SiO2 particles can be applied to account for the local strain of the rubbery matrix. Furthermore, it is believed that the damping function is a function of the localized deformation of the rubbery matrix independent of the SiO2 content. The fact that the time-strain separability holds for both the unfilled SSBR and the filled Compound indicates that the nonlinear relaxation is dominated by the rubbery matrix, and this implies that the presence of the particles can hardly qualitatively modify the dynamics of the polymer. It is thought that the filler-rubber interaction induces a coexistence of the filler network with the entanglement network of the rubbery phase, both being responsible for the nonlinear relaxation. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 3569-3574, 2009