This article examines the trading cost hypothesis by using Taiwan stock index futures and mini index futures. Empirical results show that Taiwan stock index futures, which has a lower trading cost than mini index futures, plays a dominant role in the price discovery process. The results suggest that trading cost hypothesis is supportive in Taiwan futures market. Moreover, the empirical results also find that, the institutional investors move the price of Taiwan stock index futures more than the individual investors do. In other words, the informed traders in Taiwan futures market are institutional investors, which is consistent with the findings in the European and American financial markets.