Polypropylene(PP) and polyamide6(PA6) were extruded with a single screw extruder. It was found that when PA6 was in the dispersed phase, it could deform and fibrillate in the PP matrix in the shear force field generated by the rotating screw. Dynamic mechanical analysis and mechanical tests showed that the mechanical properties, such as the tensile strength, impact strength, and modulus, were different in the different processing directions. With the addition of PP-g-MAH as a compatibilzer, the number of fibrils decreased and the dimension of the dispersed phase decreased significantly. This indicated that the compatibility has a strong effect on the in-situ fibrillation.