The functionalization reaction of high density polyethylene (HDPE) with maleic anhydride (MAH) or with MAH and gamma-methacryloxy-propyltrimethoxysilane (MAS) performed in melt state through a high shear stress-induced initiation by increasing the screw rotation speed of twin-screw extruder and through a compounded initiation by adding some initiator and increasing the screw rotation speed was investigated in this article. The results show that by increasing the screw rotation speed during melt-extruding process, the scission of HDPE chain bonds can be caused to form macroradicals, the functionalization reaction of HDPE with MAH or with MAH, and MAS can be realized. The percentage of grafting and the melt flow rate of the functionalized products depend on the screw rotation speed and reaction temperature. The cross-linking reaction during melt extrusion can be suppressed by increasing the screw rotation speed and the reaction of HDPE with MAH can also be promoted by adding a second grafting monomer MAS. The high shear stress-induced reaction products have a higher reactivity with the coupling agents coated on the surface of glass fibers and can obviously increase the mechanical properties of HDPE/GF composite. The SEM experimental results indicate that an oriented crystal transition layer exists between the interface of glass fiber and the matrix, the interfacial bonding strength is the determining factor of the formation of the oriented crystal layer. POLYM. ENG. SCI., 48:2277-2286, 2008. (C) 2008 Society of Plastics Engineers