To investigate the effect of block copolymer addition, as a compatibilizer, on the phase separation behaviors of the immiscible polymer blend, Polystyrene(PS) and Polyisoprene(PI) were chosen as model compounds for a binary blend pair, and Poly(styrene-b-isoprene) (SI) as a compatibilizer. Using a time-resolved light scattering apparatus, the phase separation kinetics were investigated for the blends with or without block copolymers. The block copolymer, which shows the compatibilization effect, reduced the interfacial tension between the continuous medium and separated domains, thus making the size of domains be decreased and the separation rate be retarded. To examine the dependancy of the molecular structure of block copolymer on the compatibilization, various types of SI's differing in total molecular weight were employed. For a series of low molecular weight SI's whose microphase separation temperatures are lower than the experimental temperature, there was no compatibilization effect. It was also found that there were good agreements, even though in a semiqualitative sense, between the experimental results and theoretical prediction proposed by Lifshitz and Slyozov.