This study investigates the impact of the trading of the national pension fund (NPF, hereafter) on the stock markets in Korea. In 2017, the Korean national pension system, which was introduced in 1988, was ranked as the third largest public pension fund after Japan and Norway, with assets of 6.8 trillion Korean won. Moreover, the NPF accounts for 6.75 percent of the Korean equity market, which is higher than the 5.06 percent of equity held by the Japanese pension fund. As the NPF has increased its investment in the Korean stock market, its trading strategies and possible influence on the stock market have attracted increasing attention from market participants and regulators. However, despite the growing interest in the subject, few studies have examined the strategies and influence of the NPF and there is little research data on the trading in the past decade. In addition, the NPF recently announced that it plans to increase the proportion of investment in stocks in its portfolio over the next five years. Thus, the NPF is expected to have an increasing influence on the Korean stock market. Given the significant size of the NPF, the increased proportion of stocks held by the fund could lead to price pressures unrelated to the intrinsic value of firms and result in price movements induced by follow-up investors. For these reasons, there has been considerable debate over the effect of the NPF's trading on the stock market. Some have argued that the NPF's investment decisions are based on political judgement rather than information whereas other institutional and foreign investors conduct information-driven transactions. In line with this, the NPF recently provided an official explanation of why its stock portfolio only achieved average returns of 5%, which is significantly lower than the 22.15% average returns on the Korea Stock Exchange (KSE) and 24.75% returns on the Korea Securities Dealers Automated Quotation (KODAQ) market. Taken together, these concerns suggest that there is a pressing need to comprehensively examine whether the NPF's transactions on the stock markets distort the markets and worsen the market quality in terms of liquidity and volatility. In this study, we attempt to address these questions by examining the influence of the NPF's transactions on the Korean stock markets. Our dataset comprises the combined data from the ownership change and trading records of the KSE and KODAQ over the 12-year period from August 2005 to July 2017. Specifically, we combine the insider equity ownership disclosures, disclosures of large equity ownership, and information on the trading record files and infer detailed information on the dates, quantity, and prices of the stocks traded by the NPF. Based on the inferred accounts of the NPF, we analyze the trades of the NPF from 2005 to 2017 at both the aggregate market and individual stock levels. Some of our major findings are as follows. First, consistent with the literature, we find that the NPF uses a contrarian trading strategy such that the fund tends to buy stocks when the market return goes down and sell them when the return goes up. This phenomenon is also observed at the individual stock level even after controlling for various stock characteristics, and the results are consistent across the yearly analyses. Second, the market volatility decreases when the NPF's net buying increases. Therefore, the NPF's stock transactions do not appear to destabilize the stock market. Furthermore, in contrast to the general concerns, our empirical results suggest that the NPF's stock transactions contribute to market stabilization. Third, we do not find evidence showing that the trading of the NPF influences the trading of other investors, including the other sub-groups of institutional investors and individual investors. However, we find that among the various sub-institutional investor groups, the increased net buying of the NPF is associated with increased net buying by investment companies and miscellaneous financial investors. These sub-groups are influenced by the net buying of the NPF after two to three days, indicating follow-up transactions. Together, these findings suggest that some financial investors may consider the NPF to possess a strong capacity to analyze information.