This paper shows that an AT-cut quartz crystal analyzer can be applied to the phase change study of polymer blend thin films. The phase trasition and phase separation phenomena of blend films of poly (methyl methacrylate) (PMMA) and poly (vinyl acetate) (PVAc) cast from ethyl acetate have been investigated by quartz crystal analyzer(Q.C.A.). The resonant frequency and resonant admittance are measured simultaneously and the mass change and viscoelastic change of the polymer blend thin films are analyzed.