The study finds that IPO underpricing premiums in Taiwan stock market become significantly larger when IPO firms are required to offer new shares instead of existing shares to investors beginning in 2005. The result suggests that the new rule provide room for IPO firms to lower their offering prices. The study makes an effort to explain changes in insiders’ holdings during the post-IPO period. We also find that the explanatory power of factors such as underpricing premiums and post-IPO returns increase when we adjust the underpricing premiums and the post-IPO returns by subtracting from the market performances to obtain the abnormal underpricing premiums and the post-IPO returns.