Mechanical properties such as tensile strength, tensile modulus, Izod impact strength, flexural strength, flexural modulus, heat deflection temperature and abrasion resistance of ABS and Nylon composites filled with potassium titanate whiskers were investigated with respect to the effect of filler content, surface treating agent and grafting modification of filler The surface treating agent used in this sutdy was KBC-1003 silane coupling agent (with vinyl silane). For tensile and flexural properties of ABS and Nylon filled with whiskers either surface treated with silane coupling agent or untreated, the treated whisker was more effective in the filling effect than the untreated whisker due to the increase in interfacial adhesion between the filler and the matrix. Particularly the highest was obtained when the composites were filled with styrene grafted potassium titanate whiskers. In addition, the effectiveness of various modifications in enhaning the Izod impact strength was in the following order: styrene grafted potassium titanate whisker filled composites >MMA grafted potassium titanate whisker filled composite >coupling agent treated potassium titanate whisker filled composite >untreated potassium titanate whisker filled composite. The heat deflection temperature, when ABS and Nylon were filled with potassium titanate whiskers, was increased slightly. The abrasion resistance, however, showed remarkable increase when ABS and Nylon were filled with surface treated potassium titanate whiskers.