Abstract: The silane grafting and crosslinking high density polyethylene (HDPE) filled with MgO was prepared by the method of two-step reaction-extrusion, in order to study the effects of the silane grafting and crosslinking on the crystallinity, the mechanical properties, the heat resistance, and the thermal conductivity of the MgO/HDPE composites. And the graft-crosslinking and the crystallization were characterized by FTIR and DSC, respectively. The results show that the tensile strength, the notched impact strength and the heat deflection temperature (HDT) of the MgO/HDPE composites increase from 20.41 MPa, 1.875 kJ/m² and 74.6℃ without graft-crosslinking to 27.24 MPa, 7.875 kJ/m² and 83.5℃ respectively after crosslinking, and the thermal conductivity of the crosslinking composites (0.447 W/(m·K)) is almost the same with the non-crosslinking ones.