It has been considered that the higher the stake of the largest shareholder, the higher the possibility of a company’s opportunistic behavior. However, the prior studies have not succeeded in showing consistent results between the largest shareholder’s stake and accounting numbers. Therefore, this study empirically tried to examine the effect of the largest shareholder’s share on quarterly earnings quality depending on whether the largest shareholder directly manages the business or not. Quarterly accruals quality was measured based on the Francis et al. (2005) model and then, it was used as a proxy for quarterly earnings quality. As a result of analyzing 14,060 company-year-quarter data of KRX-listed companies from 2015 to 2021, it showed that the higher the share ratio of the largest shareholder, the quarterly earnings quality would deteriorate when the largest shareholder participates in management. Additional analysis reflecting the non-linear relation of the largest shareholder’s ratio to the accounting numbers also showed that the quarterly earnings quality was more affected when the largest shareholder directly participated in management. In addition, it was confirmed that the largest shareholder’s opportunistic behavior was more prominent in the fourth quarter when earnings management motives were expected to be higher than in other quarters.