The purpose of this paper is to examine the determinants which cause a price differential between common and preferred stock. Prior studies have shown that variables like liquidity, size, the number of outstanding shares issued can explain the price differential between common and preferred stock price. Based on year 2006 through year 2008 data, we analyzed the determinants using regression model. Dummy variables representing large/small company and KSE/KOSDAQ respectively are added and analyzed as independent variables. The firm size, trade volume turnover, and the number of preferred shares to total outstanding shares were proved to make influence on the price differential under the 5% significance level. Especially, we have found the number of preferred shares to total outstanding shares provide the most strong relationship with the price differential. This means that a high ratio of preferred stock to total outstanding shares leads to relatively high value of common stock and causes a big price differential.