Two blend systems of polystyrene／poly (methyl methacrylate) (PS／PMMA) and polystyrene／poly(ε-caprolactone) (PS／PCL) were studied by using their thin films prepared on a glass substrate by spin-coating from THF respectively．Phase contrast microscopy and scanning electron microscopy (SEM) combined with elementary analysis were used to obtain information on the morphology in these thin films after heat treatment．It was found that PMMA-rich and PCL-rich phases were always continuous in thin films of either blends

even though the PMMA and PCL were the minor component in their blends．This should result from the better wetting abilities of PMMA and PCL on the glass substrate than PS in the blends．The spreading of PMMA or PCL phase in the blends on glass substrate during annealing induced the above results．The function of viscosity of the blend in the evolution of phase structure was also concerned via changing the molecular weight of PS component in the PS／PCL blend thin films．