Abstract: Companies with the aim of expanding income diversification do. Management strategy theory indicates that diversification may have either positive or negative effects on the quality of outcome reports. The aim of this research is to review the impact of diversity in audit companies on audit quality in companies listed in Tehran Stock Exchange. In this research, diversification and size of audit companies are considered as independent variables and audit quality as dependent variable. Companies listed in Tehran Stock Exchange comprise the research population. Through systematic screening method, a number of 181 listed companies in the 2007-2013 time span were selected and logistic regression model was used to test the hypotheses. The results indicate that there is a negative relationship between the diversification of an audit company and the audit quality. Other results indicated that the size of an audit company decreases the conflict between diversification of an audit company and the audit quality.