The mechanical properties of glass-fiber-reinforced polystyrene composites were investigated with the variation of glass fiber content, surface treatment conditions, and silane coupling agents. gamma-MPS, GPS, and poly(gamma-MPS-co-styrene) were used for the surface modification of glass fiber. Mechanical properties of glass-fiber-reinforced polystyrene composites increased with increasing the content of glass fiber. Poly(gamma-MPS-co-styrene) was superior to gamma-MPS and GPS in promoting the interfacial adhesion of glass fiber-reinforced polystyrene composites. The mechanical properties of composites were maximum at a low copolymer concentration when the gamma-MPS content in the copolymer was high and vice versa.