Compatibilizing effects of polyethylene-poly(methyl methacrylate) diblock copolymer (P(E-b-MMA)) on immiscible blends of polyethylene (PE) and poly-4-vinylphenol (PVPh) have been investigated by scanning electron microscopy (SEM) and optical microscopy (OM). In OM observation, it was found that there was coalescence of the PVPh particles in the PE/PVPh blends without P(E-b-MMA) at 180 degrees C. On the contrary, the coalescence of the PVPh particles was not observed in the PE/PVPh blends added with P(E-b-MMA) at 180 degrees C. In addition in the SEM observation, when small amount of P(E-b-MMA) are added to the PE/PVPh blends, the size of PVPh particles decreased drastically and their size distribution become narrower with increasing P(E-b-MMA) content. Therefore it has been concluded that P(E-b-MMA) acts as the compatibilizer in the PE/PVPh blends.