This paper examines the effect of auditor's quality on earnings management. There are two hypotheses about managerial discretion in the literature. One is the information-signaling hypothesis, discretionary accruals help managers to convey the private information about future profitability beyond the historical cost accounting, to improve the ability of earnings to reflect economic value. The other is the opportunistic accrual management, discretionary accruals are employed to hide poor performance or postpone a portion of unusually good current earnings to future years, and hence they are value-irrelevant. The empirical results show that higher quality auditors reduce more opportunistic accruals. However, the evidence in the paper doesn't support that audit quality can enhance the association between discretionary accruals and changes in firm's future performance. Reducing the noise of earnings is the passive effect of audit quality. On the other hand, improving the informativeness of earnings is the active effect. These two effects should be treated importantly as equal, but it seems that the passive effect is much more often discussed. The results showed in the paper can provide auditing standards setters and accounting profession regulators to rethink the contribution of auditors to the quality of financial reporting.