High-impact polystyrene (HIPS) /nano-TiO2 nanocomposites were prepared by surface pretreatment of nano-TiO2 with special structure dispersing agent (TAS) and master batch manufacturing technology. The results show that when the nano-TiO2 content is 2%, the notched impact strength, tensile strength, and elastic modulus of HIPS/ nano-TiO2 nanocomposites increased to a maximum. This result indicates that nano-TiO2 has both toughening and reinforcing effects on HIPS. The heat-deflection temperature and flame-retardance of HIPS/nano-TiO2 nanocomposites are also obviously improved as the nano-TiO2 content is increased. The nanocomposites manufactured by the two-step method have better mechanical properties than that made by a one-step method. HIPS/nano-TiO2 nanocomposites are also non-Newtonian and pseudoplastic fluids.