We studied the effects of the molecular weight distribution (MWD) on the dynamics of the early stage spinodal decomposition (SD) in poly(styrene-ran-butadiene) (SBR)/polybutadiene (PB) mixtures by using time-resolved light scattering techniques. In order to study the effects, we used four binary mixtures of SBR/PB with a given composition in which the PB components have the same weight-average molecular weight but different MWDs. The time changes in scattered intensity profiles of the four SBR/PB mixtures can be described by Cahn’s linearized theory over the range of wavenumbers covered in this study. The characteristic parameters such as the collective diffusion coefficient and the characteristic wavenumber of which the scattered intensity increases with time at a maximum rate were analyzed on the basis of the linearized theory. The characteristic parameters for the four mixtures turned out to be identical so that the dynamics was found to be insensitive to MWD, depending on the weight-average molecular weight.