The creditors, as one of the sources of finance, need comprehensive and accurate information about the company, and the primary objective of accounting is to provide information that investors and creditors can help in this. The purpose of this study was to examine the effect of audit quality on the relationship between the weakness of internal control and the cost of debt of listed companies in Tehran Stock Exchange. For this purpose, information from 121 companies listed in Tehran Stock Exchange has been reviewed during the period from 1390 to 1396. For processing the research model, ordinary least squares and multivariate regression have been used. Also, Eviews software version 9 and Excel 2010 have been used for computing The results of the research hypothesis test indicate that there is a positive and significant relationship between the weakness of internal control and the cost of debt of listed companies in Tehran Stock Exchange This means that with the increasing weakness of internal control, debt costs also increase Finally, the quality of audit has a positive and significant effect on the weakness of internal control and debt costs of companies accepted in Tehran Stock Exchange.