The overallotment system, under which underwriters are responsible for stabilizing prices of IPO shares, has been enforced in Taiwan since 2005. This research investigates the determinants of the overallotment ratio for IPOs in Taiwan. The result indicates that the overallotmemt ratio is a U-shaped, rather than linearly decreasing, function of the IPO issuing size, especially for IPOs in the OTC market. Furthermore, relative to a bear market, a bull market witnesses large overallotmemt ratios for IPOs in both the OTC and TWSE markets. Finally, the prestige of the underwriter is inversely related to the overallotment ratio for IPOs in the OTC market, while the relation is trivial for IPOs in the TWSE market.