Abstract: The in-situ microfibrillar composite based on poly (ethylene terephthalate) ( PET) and polyethylene( PE) was prepared through melt ext rusion, hot stretching and quenching process. Fixing the mass composition(15 ∶85) of the PET and PE, increasing the hot stretch ratio, the spherical and ellipsoidal PET particles t ransformto rodlike particles, finally to well-defined microfibers. During hot stretching, the maximum and average diametersof PET particles reduced steadily while the minimum fiber diameter remained constant . The PET fibers were successfully preserved in the materials while being processed at the processing temperature of PE, but they were meltedagain and deformed into spherical particles at the processing temperature of PET. The tensile modulus and st rengthof the microfibrillar composites are significantly enhanced with increasing the hot st retching ratio, indicating that themicrofibers have a good reinforcement on the PET/ PE composites. The ultimate elongation is greatly decreased withhigher hot stretching ratio and there is a critical hot st retching ratio, at which the ductile-brittle t ransition occurs.The specific essential work of f racture ( we ) of the composite first increases with the increase of the hot stretch ratio,and shows a maximum at a hot st retch ratio of 19117, then decreases with further increasing the hot st retch ratio.