After Asian financial crisis in 1998, the regulation of stock trading against foreigners in Korea have been greatly released so that the foreigners' ownership of the whole Korean stock market in the late 2000s has increased around 40% from 10%. That change in investor constitution might have an effect on the dynamics of stock prices in the Korean market. Given the circumstance, this paper examines whether the momentum effect comes into being as a result of the increase of foreign investors' trading. In the results, the simple zero-cost portfolio with buying past six month winners and selling past six month losers does not show significant profit. However, we observe the significant momentum profit of the zero-cost portfolio composed of large firms. Since foreigners mainly invest in large firms in the Korean stock market, that momentum profit can be caused by foreigners. In the study, this presumption is supported by the analysis of price contribution on the zero-cost portfolio and the proportion of winners and losers in the portfolio classified by firm size and foreign ownership changes.