This study examines the association between professional training, higher academic qualifications (educational levels) and operating performance of audit firms in Taiwan. We particularly focus on the curvilinear effects of higher academic qualifications on operating performance. We group the total sample into three categories: national, regional and local audit firms. Based on the theoretical framework in industrial economics, we establish a cross-sectional multiple regression equation to test our hypotheses. Both higher academic qualifications and professional training are positively related to the operating performance of audit firms. Professional training moderates the relation between higher academic qualifications and operating performance. Higher academic qualifications exhibit a curvilinear effect on operating performance with a reverse U-shaped relation for the national audit firms and a U-shaped relation for both regional and local audit firms. Due to data unavailability, some factors affecting the audit quality and operating performance are not included in our analysis, such as auditor teamwork, internal control system, operating policies and auditing procedures of audit firms. The findings that higher academic qualifications are positively associated with the operating performance of audit firms justify the educational policy of establishing institutes or graduate schools in accounting over the past two decades. Furthermore, audit firms skillfully exploit employees with higher academic qualifications to improve their operating performance. We are the first to document the moderating effects of professional training and the curvilinear association between higher academic qualifications and operating performance, contributing knowledge to related literature.Sustainability 2020, 12, 1254 2 of 16 reorganization of many junior colleges as an academy or technology university. The sharp increases in the number of university lead to substantial growth in the number of graduates with higher educational levels. Taking the higher education of accounting as an example, to date, 37 universities establish departments of accounting to grant bachelor's degree. Among which, 26 universities set institute or graduate school in accounting to deliver master or doctoral degree. Under the landscape of educational policy, Taiwanese audit firms recruit more auditors with higher educational levels. The Taiwanese Financial Supervisory Commission indicates that the ratio of auditors with bachelor's degrees rose from 49.3% in 1992 to 66.8% in 2015. The ratio of auditors with master's degree or above was 7.6% in 1992 and climbed to 20.4% in 2015. The growth rates of auditors with bachelor's degree are 35.5% but that of auditors with master's degree or above are as high as 168.4%.Audit firms are a professional organization and an expertise and human capital-intensive industry. To fulfil their auditing jobs, auditors are required to have academic educational levels, professional training and work experience [4]. Auditors typically in...