Microstructural deformation behavior of polystyrene(PS) /polyphenylene oxide(PPO) and PS/polyvinylmethylether(PVME) was analyzed using transmission electron microscope(TEM) and microdensitometer. Individual craze and shear deformation microstructures were observed and quantitatively analyzed in terms of its characteristic fibril extension ratio(L) using microdensitometry. Microdensitometry results showed that L values decreased as the PPO and PWE content increased. These behavior can be qualitatively explained using the concept of chain entanglement in PS/PPO blends. However, in PS/PVME blends, these L behavior can be explained using the intermolecular interaction induced relaxation behavior of PVME chains.