The in situ composite consisting of polypropylene (PP) and polyamide 6 (PA6) was obtained with extrusion method in a single screw extruder. It was found that PA6 being the dispersed phase could fibrillated in PP matrix and fibrillation occurred in the channel of the extruder. The notched impact strength increased with the increasing content of PA6 while the tensile strength decrease. The average diameter of the fibrils changed hardly with the changing content of PA6 under the investigated condition. Increasing the draw rate and the impact strength and tensile strength increased obviously. When increasing the rotating speed of screw the dimension of the fibrils also became finer. The mechanical properties decreased with the increasing processing temperature and the average diameter of fibrils increased.