A compound containing 75 wt% of high-density polyethylene (HDPE) and 25 wt% of polypropylene (PP) was melt-mixed in an extruder. The resulting compound was fibrillated by drawing it through a pair of steel rollers at 138 degrees C. The fibrillated tape was cut into small pieces and then re-molded at temperatures above the melting point of HDPE. TEM results indicate that PP fibrils of about 30-150 nm in diameter were created in the HDPE matrix. The mechanical properties of HDPE were greatly improved with a yield stress above 60 MPa and a Young modulus of 3.5 GPa.