The present research aims to evaluate the role of tax audit in tax system and reduce the expectation gap between auditors, legal entities and tax organization. Different indicators can be used to evaluate the role of auditing in the tax system. Here four dimensions have been used to determine audit expectation gap which include consideration of accounting standards and tax regulations, time intervals between: assessing and issuing notice of assessment, notice of assessment and final assessment, final assessment and collection and enforcement. Statistical population of this research is legal entities of west Tehran tax offices who welcomed tax audit for over 5 consecutive years 2002-2006. Required information has been acquired by document mining method (real data on tax files). T-test and t-value has been used for testing research hypotheses. According to this research, tax auditing accelerates tax collection, reduces tax lags and enhances the quality of tax reports by taxpayers.