With the boosting in funds investment, the study issues regarding investment style are followed with interest. Consequently, this study further extensively analyzes the impact relationship between investment style and investment performance for Taiwanese stock mutual funds. To this end, this study uses panel data regression to analyze the relationship between investment style variables considered in this study and investment performance and aim for finding the key impact variables. Besides the overall period, the bullish period and bearish period are also examined. The empirical study evidently indicates that the moderately distributed and unbalanced allocation among stocks and industries in bullish period may get the better investment performance. While, it should be preferable to extremely concentrated and balanced allocation among stocks and industries in bearish period. On the other hand, the investment performances of those funds, which hold a majority of higher beta and/or lower EPS stocks, are superior in bullish period, whereas an opposite outcome is found in bearish period. Additionally, the P/B ratio appears a significantly positive impact on the investment performance regardless of bullish or bearish periods. It implies that Taiwan investors commonly prefer the growth investment to the value investment. By contrast, the capital scale is lesser significance, but a negative effect is constantly revealed through overall period. Middle and small companies are slightly prominent to large companies in the performance of stock price. These empirical findings should render a valuable reference for fund managers in forming investment style and for investors in seeking out the stock mutual funds.