If we can estimate the level of private information that has flowed into the capital market as a statistic, it can be very useful in many related fields. The first example is the probability of information-based trading (PIN) of Easley and O'Hara (1987). However, the structural model, PIN, has been criticized since the mid 2000s for not reflecting the data characteristics of the stock market, which has been turned into a surge of high frequency trading (HFT). Then, Duarte and Young (2009) proposed a modified PIN model, which extends the PIN model to overcome its shortcomings, and Odders-White and Ready (2008) suggested a model which is based on a fully different theoretical approach. This paper examines the validity of PIN, adjusted PIN (AdjPIN), and Odders-White and Ready (OWR), which have been extensively used as private information measures, in the Korean stock market. Applying them to the information asymmetry situation of the securities market before and after the merger announcement, we verify whether the estimates of these models are appropriate as private information measures. For this, we use the “Conditional Probability of an Information Event (CPIE)” of Duarte, Hu, and Young (2015, 2017). CPIE is estimated by model to enable to judge whether the private information reaches the market on a daily basis. The analyses in this paper include 29 cases of merger disclosures for the targeted companies listed on the KOSPI Market over an eight-and-a-half year period (2007. 5~2015. 10). We employ regression analysis, comparison between CPIE and its expected value, and the CPIE determinants analysis using the TAQ data for a total of 340 trading days before and after each event. As a result, only the OWR model among the three models appears to be useful as a measure of private information in the Korean securities market. Although there are differences in detail, this is essentially the same result as the US stock market. It suggests that in the Korea stock market, the OWR model based on orderflow imbalance, and day and over-night returns, is more appropriate as a measure of private information, rather than the PIN / AdjPIN model. The more detailed results are as follows. First, as a result of CPIE determinants analysis, the PIN-model-based CPIE is affected mostly by trade turnover and somewhat by order imbalance, while the AdjPIN-model-based CPIE is affected by trade turnover. Accordingly, the PIN/AdjPIN models are affected mainly by trade turnover, rather than order imbalance, on which they are theoretically based. On the other hand, most of the explanatory power of the OWR-model-based CPIE comes from over-night and intraday returns. The OWR-model-based CPIE reflects information consistent with its theoretical rationale, although it contrasts somewhat with the U.S. stock market, which is much more affected by either overnight return (in regression analysis) or intra-day return (in the comparison analysis between CPIE and its predicted value). Second, the PIN-/AdjPINmodel- based CPIEs are estimated at the boundary values 0 or 1 at least 81% of the time due to numerical overflow problems in the parameter estimation. This makes PIN or AdjPIN measurements more difficult to trust, even if the models are valid. Third, in all three models, the CPIE persists for up to 5~6 trading days after the announcement of the merger. However, after the announcement, the pattern shows a gradual decline to a lower level than before the announcement. Also, in all three models, the CPIE peaks one day after the announcement, rather than on the day of the announcement. This reflects a degree of inefficiency in the Korean stock market. Altogether, these results suggest that, among the PIN, AdjPIN, and OWR models, the OWR model best reflects the private information flow in the Korean stock market. In other words, they suggest that the OWR model based on order flow imbalance, over-night return, and intraday return, rather than the PIN/AjdPIN models based on order flow imbalance, is more appropriate as a measure of private information. The results on the Korean stock market are essentially the same as those of the U.S. stock market, though there are differences in detail, such as the relative importance of overnight and intraday returns.