<p>High productivity as well as high product quality is generally required in plastic products. Vibration-assisted plastic injection molding (VAIM) is an attractive injection molding for high product quality. The major process parameters in VAIM are the amplitude and duration during the vibration, but they are conventionally determined by the trial-and-error method. Long duration results in low productivity, whereas short one leads to low product quality. In addition, other process parameters such as melt temperature, cooling time, cooling temperature also affect the product quality and the productivity. Therefore, it is important to determine the optimal process parameters in the VAIM. This paper proposes a method to determine the optimal process parameters in VAIM for simultaneously minimizing the warpage and the cycle time of a thin-plate product. The numerical simulation in VAIM is computationally so expensive that sequential approximate optimization that response surface is repeatedly updated by adding sampling points is adopted, and thus the pareto-frontier between the warpage and the cycle time is identified. The experiment using the VAIM is also conducted using the injection molding machine (MS-100, Sodick). It is shown through the numerical and experimental result that the VAIM effectively improves the warpage and the cycle time.</p>