This study focuses on the effects of the market making regulations in Korea on the excess returns from IPO stocks with put-back options, using stock market data of firms listed on the Korea Exchange(KRX) including both KOSPI and KOSDAQ markets during the period starting from September, 2003 from which the put-back option were imposed to the underwriters in Korea till the end of 2010. Under the regulations during the period of market making, the investors into IPO stocks had the right to sell their stocks back to underwriters of IPO stocks within the first month of trading at the 90% of the offering price. Such put-back options attached to the IPO stocks are expected to increase the value of the stocks because they protect investors from potential losses due to asymmetric information between underwriters and/or IPO firms and investors or moral hazards of underwriters when their proper market prices were not in existence in the regular market. While most past studies in this field are on the underpricing of IPO stocks during some days after their listing, this study focuses on the effects of put-back options by comparing the stock market performances of such firms during the period of market making with put back options and with those without such options. We also study such effects by separating them into two parts: the underpricing of IPO itself which is measured by using differences between the initial price of public offerings set by the underwriters and the starting price of IPO stocks on the first day of trading and that after the IPO measured by the excess returns in some days. Main findings of this study are as follows: First, the market making system introduced for some years in the Korea Exchange lead to underpricing of the IPO stocks. Before the new regulations on the underwriting of IPO stocks in Korea in August, 2007 during which the put-back options were imposed to underwriters, the IPO stock price on average had been discounted as much as 20.9%. The result supports the claim by the Financial Supervisory Service of Korea that the new system of IPO introduced on May 15, 2007 lead to underpricing of IPO stocks due to market making policies included in the regulations. Second, the effects of IPO underpricing lasted only up to two weeks at the longest. This might imply that investors into IPO stocks had better sell them early enough to realize their excess returns due to capital gains, possibly the best from selling on the day of the initial listing. It is also recommended from the findings of this study that investors in reality might suffer from losses on average after a week after the initial listing, which is supported by the phenomena that are observed in the stock market where institutional investors which participated in the IPO sell off more often than not IPO stocks on the day of listing or not long before after the IPO. Third, this study in most cases provides the evidence of positive effects of put-back options on market making through higher excess returns during the period when put-back options were imposed by the law. Such positive effects lasted for some days after the listing day. However, the effects were declining more rapidly for the period of market making with put back option clauses in the contracts for investors than for that without such options when the price declined below 90% of the offer price within one month period after the initial offering. This result might imply that stocks more highly underpriced can be performing much worse later after some days after IPOs than less underpriced stocks, and that investors are better off with earlier sell-off of the IPO stocks as early as possible after the IPO when sufficiently high underpricing effects could be realized on the day of IPO.