The measurement of physical properties of thermoplastic natural rubber (TPNR) of NR-HDPE blends have been made at various compositions of high-density polyethylene (HDPE). The tensile properties and hardness of TPNR improve significantly with the addition of liquid natural rubber (LNR) to the blend. The degree of cross-linking also increases with increasing amount of LNR added. The LNR with molecular weight (M(w)) of 50,000 and reactive terminals promotes cross-linking within the rubber phase and grafting of the polyethylene chains onto the rubber matrix system. The maximum stress and strain of the blends measured are about 7.5 MPa and 1000%, respectively. Dynamic mechanical analysis results indicate a single T-g on a tan delta trace at about -50 and -55 degrees C for the two types of blends, respectively.