Formula for glass fiber/microcellular unsaturated polyester composites (GF/MCUP) using supersaturated gas technology was studied by the way of orthogonal experiment. The results showed that these factors affecting average cell size from strongly to weakly were styrene content, accelerant content, glass fiber length, curing agent and glass fiber content, respectively. The factors affecting average cell density from strongly to weakly were accelerant content, styrene content, glass fiber content, curing agent content and glass fiber length, respectively. Curing agent content was the major factor affecting impact strength of GF/MCUP. The factors revealed no obvious difference in affecting tensile strength. Under the optimizing processing condition, the average cell size of GF/MCUP was about 8 μm and the cell density was 1.19×109 cells/cm3. The 153.70 and 255.84 % increase in impact strength were gained over that of GF/UP and UP, respectively. The corresponding 20.24 and 82.51% increase in tensile strength-to-weight ratio were gained over that of GF/UP and UP, respectively.