Using Taiwan listing companies from 2010 through 2016 as sample, this study aims to examine the association between client importance and tax avoidance. To ensure that the empirical results are not affected by a single event during a given fiscal year, the degree of tax avoidance is respectively measured by the three-year effective tax rate, the three-year current tax rate, and the three-year cash effective tax rate. In addition, an accounting firm's client importance is measured by the audit fee paid by client in proportion to the aggregate audit fees accounting firms receive from all Taiwan listing companies. Regardless of whether the accounting firm or engagement partner is used as measurement basis, the empirical results indicate that the higher the accounting firms' (the engagement partners') economic dependence on the audit client, the lower the degree of client tax avoidance.