Novel interpenetrating polymer networks (IPNs) of Polydicyclopentadiene/Polystyrene (PDCPD/PS) were prepared by in-situ polymerization using Ziegler-Natta and peroxide as double catalytic systems. The structure of PDCPD/PS alloy was characterized by Soxhlet extraction, infrared spectrum (IR) and scanning electron microscope (SEM). The results showed that the polymerization of dicyclopentadiene (DCPD) was facilitated in the presence of styrene and the rate of polymerization as well as the conversion of PDCPD was improved with styrene content. The strong IR absorption peaks of PS indicated high penetration of PS within PDCPD networks, and in the process forming IPNs. There were no obvious traces of PS in the alloy brittle fracture, which prepared in liquid nitrogen and was etched by toluene. This result also demonstrated that PS was dispersed in molecular level in the polymer alloy networks and it tangled with PDCPD. The measurements mechanical properties showed that the tensile strength of the polymer alloy was improved with styrene content. This may be due to homogeneous dispersion of PS in the IPNs. The synergistic effect of PS and PDCPD also played a part in enhancing the mechanical properties of polymer alloy.