Casein was grafted with acrylonitrile (AN) and with a binary mixture of acrylonitrile and n-butyl methacrylate (n-BMA). Stress-strain characteristics of these films were analyzed as a function of temperature and rate of extension. The results shows that both the systems are sensitive to temperature. Due to the incorporation of n-BMA, the material becomes flexible since the T(g) of the multiphasic polymer was reduced and therefore shows the changes even at lower temperatures when compared to casein grafted with AN. The fracture morphologies of the binary mixture-grafted casein films show that the effect produced by temperature and rate of extension are inversely related.