This study investigates the effects of nipa palm fiber on Acrylonitrile Butadiene Styrene (ABS) composite and the application of styrene polymerization on the mechanical properties of the achieved composite materials. ABS sheets prepared from its pellets by using hot-pressing method was used to produce nipa palm fiber-reinforced composite. In addition, styrene polymerization was applied to increase the interfacial strength between fibers and matrix. The mechanical properties of these two composites were tested for comparison. The use of nipa palm fiber reinforcement for composite materials greatly affects the mechanical properties of the material. Compared with the sample of 100% resin, the impact strength is lower; however, the tensile and bending increases. Styrene polymerization increases the tensile comparing with the results of untreated fiber samples. This could be explained by the induced polystyrene acting as a bridge link between fiber and resin base. This leads to the increase in durability link and mechanical properties of the composite sample.