Present manuscript deals with surface modification of Grewia optiva fiber through graft copolymerization with acrylonitrile (AN). These surface modified cellulosic fibers were then used as reinforcement to prepare natural fiber reinforced unsaturated polyester matrix based biocomposites. Different reaction parameters for graft copolymerization were optimized to get the maximum graft yield (10.42%). The graft copolymers formed were characterized by FTIR, SEM, XRD, and TGA techniques. On grafting, thermal stability of fibers increased, which has been also supported by energy of activation (Ea) data obtained for both raw and grafted fiber. The grafted fibers were also subjected to evaluation of physicochemical properties. Subsequently, composites of unsaturated polyester matrix reinforced with different fiber loadings (0, 10, 20, 30, and 40% in terms of weight) of both raw and grafted fiber have been fabricated by hand lay up method. Mechanical properties of composites were studied and have been found to be higher when compared with polymer matrix. However, maximum enhancement in mechanical properties has been observed in case of grafted particle reinforced UPE composites in comparison to raw particulate reinforced composites. Further these composites were also subjected to SEM, DSC, and chemical resistance studies. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers