The effect of molecular weight on dynamical process of spinodal phase separation of po-lystylene(PS) with carboxylated poly(2,6-dimethyl phenylene oxide) (C-PPO) has been investigated by using time resolved light scattering technique. In the early stage, the effect of molecular weight on dynamical parameters of phase separation, such as the growth rate, R(q) of phase se paration and apparent diffusion coefficient, are distinct, As the molecular weight increasing, the apparent diffusion coefficient, Dapp,decreases. Experimental results obtained from three mixtures tend to support the relationship of q2m/Dapp proportional to the degree of po-lymerization (N),and the diffusion behaviour coincides the reptation mechanism.