This thesis explores the impact of short selling on price efficiency for the compositions of Taiwan 50 Index during the period from 2006 to 2012.This study uses three indicators to measure price efficiency, and short selling is divided into general investor’s short selling and institutional investor’s short selling according to different type of the traders. The empirical results show that institutional investors are informed traders. When institutional investors take more short selling, it will facilitate stock price return to fundamental value. In other words, the price is more efficient. However, general investors take short selling do not significantly improve price efficiency. In addition, when investors take short selling for large stocks, it will help to reduce the extent of price discreteness, making the price more efficient; when the transaction costs are lower, it will help to enhance price efficiency.