Taking types of audit opinions as proxies for audit quality, and business revenue, CPA numbers and personnel numbers of accounting firms as proxies for auditor size, respectively, this paper tests the relationship between accounting firm size and audit quality by using the sample of listed companies of China from 2007 to 2008. It is found that there is no significantly positive relationship between accounting firm size and audit quality no matter how accounting firm size is measured. In fact, there’s an inverted U-shaped relation in general between audit quality and accounting firm size. In sum, audit quality may not increase with the growth of accounting firm size.