The information on customer concentration is not only useful for a company"s financial strategy, but also useful for investor"s corporate valuation and investment strategy. This study empirically analyzed the effect of customer concentration on the earnings management, volatility of stock returns, and beta for manufacturing companies listed in the KOSPI market and the KOSDAQ market from 2011 to 2017.*br*The main analysis results are summarized as follows. First, discretionary accruals are used to decrease profits as the sales concentration increases. Second, firms with concentrated customer bases experience higher stock return volatility. Third, positive relation between customer concentration and a supplier’s beta. This is the first study to conduct an empirical analysis using the data on sales concentration of major customers sine the introduction of K-IFRS in 2011. And this study empirically confirmed the relationship between the sales concentration of major customers and earning management, volatility of stock price returns, beta. In addition, the results suggest that the information on the sales concentration of major customers is useful information for risk assessment and valuation, and it is necessary to improve the disclosure information of the sales concentration of major customers in the future.