During the design of a two-cavity injection mould for router covers, two plans, single-gate plan and dual-gates plan, were setup based on the results of best gate location analysis combined with other aspects. Then fill analysis for the two plans were carried out. The dual-gates plan was selected as an optimal choice for its smaller clamping force, wall shear stress and injection pressure in comparison with the single-gate plan. Although weld lines and air trap exists between the two gates, but they can be eliminated or just have a little effect on the quality of parts. It indicated that the technology of mold flow analysis can be used to optimize the design of gate location and gates number and assist the engineers to improve the quality of their designs of mould.