The optimum mold design and the optimum process condition were constructed upon executing process simulation of rubber injection molding with the commercial CAE program of MOLDFLOW (Ver. 5.2) in order to solve the process-problems of K company relating to cracks, which occurs at the inner cavity wall of C. V. joint boots. As a result it was confirmed that the real cracks occurs at the exactly same position of the cavity as exhibits the defects of weld and meld line and unsatisfactory curing according to the result of simulation. In order to prevent the occurrence of weld and meld line at the defect-position, the location of gate was altered to the optimum position of the cavity. Consequently the filling pattern was established to minimize the degree of the melt-fronts confronting or the melt-flows melding to prevent the occurrence of weld and meld line at the defect-position. It was observed that both gate-positions to maximize the degree of the formation of weld and meld line and air traps are located, respectively, in opposite direction each other with reference to the optimum gate position. In addition, the temperature of mold was raised by 10 o℃ and maintained at 170 ℃ for satisfactory curing.