Polydicyclopentadiene (PDCPD)/montmorillonite Clay (MMT) nanocomposites were synthesized through reaction injection molding with a Gusmer-Decker machine by in situ polymerization. The results of X-ray diffraction (XRD) diagrams of the nanocomposites showed that MMT were completely exfoliated within the range of 0.25% to 4% MMT. The erosion wear behaviors of PDCPD/MMT nanocomposites were studied using an MCF-30 water and sand erosion wear tester. The worn surface of PDCPD/MMT nanocomposites was exami-ned with a scanning electron microscope (SEM) and the wear mechanism was discussed.It was found that when the content of MMT is 0.25%, the erosion rate of nanocompositesreach the minimum; the erosion rate of nanocomposites increases with the increase of MMT content compared with neat PDCPD. The important wear mechanism of neat PDCPD was tearing and ploughing-cutting, And the results showed that wear mechanism of PDCPD/MMT nanocomposites was dominated by ploughing-cutting, with the increase of MMT the severe mechanical cutting, fatigue wear and slight brittle rupture appear. PDCPD/MMT nanocomposites with a lower content of MMT were thus a promising structural material candidate for engineering application.