Studies on the nonlinear viscoelastic behaviors of nylon1212 toughened with styrene-[ethylene-(ethylene- propylene)]-styrene block copolymer (SEEPS) were carried out. The linear relaxation curves at relatively low shear strains show good overlap, the relaxation time and modulus corresponding to the characteristic relaxation modes were also acquired through simulating the linear relaxation modulus curves using Maxwell model. The nonlinear relaxation curves of nylon1212 blends at different shear strains have been obtained and their damping functions were evaluated. Meanwhile, it is found that most blends in the experimental windows follow the strain-time separation principle and Laun double exponential model can predict damping curves well. The successive start-up of shear behavior was investigated. The results showed that Wagner model, derived from the K-BKZ (Kearsley-Bernstein, Kearsley, Zapas) constitutive equation, could simulate the experiment data of nylon 1212 blend with 10 wt % SEEPS well, but there exists some deviation for experiment data of nylon1212 blends with high SEEPS concentrations. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 123: 1283-1292, 2012