We analyze cause and effect of Korean institutional investors’ selling behavior of IPO shares right after initial trading. Most of IPO shares sold by institutions right after initial trading are assumed to be originally allocated to them in bookbuilding process. Thus, institutions’ selling behavior of IPO shares right after initial trading is interpreted as IPO flipping that is reported by Boehmer, Boehmer, and Fishe (2006). Following the methodology of Boehmer et al. (2006), we separately analyze the initial flipping on first three trading days after IPOs, and the second flipping thereafter to the twenty fifth trading days. Samples of our study cover firms that undertook IPOs in year 2003 through year 2014 in Korea. Firms’ financial data, stock prices, trading volumes, and market indexes are obtained from FnData Guide. IPO-specific data, such as bookbuilding results, allocation ratios, etc. are obtained from investment prospectus reported in DART (disclosure system of Financial Supervisory Services). Our samples covers 674 firms: 110 firms of Securities Market IPOs, and 564 firms of KOSDAQ IPOs. We have found that IPO initial returns are significantly negatively related to the first flipping, but significantly positively related to the second flipping. This result implies that Korean institutional investors that are allocated with IPO shares take profit in the second flipping. We have also found that the main players of short-term sales of IPOs are institutional investors. The voluntary lock-up of institutional investors does not play a meaningful role in constraining their selling behavior of IPO shares. Right after the voluntary lock-up period, institutional investors are selling IPO shares. The initialflippingon the first three days after IPOs are not related to the amount of institution allotments. We have also analyzed the effect of venture capital investment on the IPO stock return of the first trading day. Empirical results show that IPO firms with venture capital investments have significantly lower stock return on the first day of trading than those with no venture capital investment. This result would be interpreted as the certification effect of venture capitals. Institutional investors tend to sell more IPO shares both in the first and second flipping when firms have venture capital investments. In the analysis of long-term return behavior, we have found that the first flipping of institutional investors does not affect long-term market-adjusted returns of IPO shares. However, the second flipping activities of institutional investors significantly decrease long-term returns measured by market adjusted returns over 3, 6, and 9 month period after IPOs. This result implies that institutional investors allocated with IPO shares tend to flip shares right after IPOs regardless of long-term prospect of IPO firms. In contrast, the second flipping is mainly driven by venture capitals that have invested in IPO firms before these firms go public. Noting that the second flipping is negatively related to long-term return performance, venture capitals that have long relationship with firms have the ability to predict future operational prospect of IPO firms. The more shares are allocated to institutional investors, the worse are market-adjusted returns over the period of 6, 9, and 12 months after IPOs. This result is related to the dynamic information acquisition model of Benvenist and Spindt (1989). In a situation where IPO allotment ratio among investor groups are mostly fixed as in Korea, significant underpricing would be an effective way of compensation for institutional investors in the dynamic information acquisition model of Benvenist and Spindt (1989). Thus, when more IPO shares are allocated to institutional investors, those shares are more likely to suffer from underpricing.