We investigate whether overnight information could affect the close-to-open returns and opento- close returns differently. S&P 500 index returns are used as overnight information that can affect the open prices of Korean stocks. The average variance ratio of open-to-close return to close-to-open returns is greater than one. This result suggests that overnight information such as S&P 500 index return seems to impact all Korean equities. Our finding is complement to Ronen (1998 ), where find that there is no significant difference in return volatilities due to the change of trading mechanism in the case of Tel Aviv stock exchange. Secondly, we examine how Korean stock returns respond to the ups and downs of yesterday S&P 500 index. We find that the close-to-open returns (Ct-1Ot) of Korean stocks are positively associated with yesterday S&P 500 index returns. On the contrary, the negative correlation appears between yesterday S&P 500 index returns and open-to-close returns (OtCt). The negative relation, as Park and Yi (2011 ) argue, is possibly due to the mispricing of investors. However, our findings are based on the prices of Korean equities, not KOSPI index returns. Thirdly we investigate whether the overnight (close-to-open) returns of Korean equities are different depending on firm’s oversea business. Companies who did not have any oversea business also responded negatively to the returns of S&P 500 indexes in during a day and positively during overnight. The relation of the stock returns of companies without oversea business and S&P 500 index returns can be understood by Barberis, Shleifer and Wurgler (2005 ), where argue that when investors price assets a common subset of information, news about one asset affects the other asset’s prices; i.e. asset price commove.