A tax audit is a tool for tax authorities to verify the accuracy of taxpayers' income and deductions. This research aims to measure the impact selfimposed or voluntary tax audit on individual taxpayer reported income using difference-in-differences (DiD). We compared the annual taxpayers' reported income before and after-tax audit in 2012. The results show while the effect of a voluntary tax audit is negative for the lowest income bracket, it is significantly positive for the higher income bracket. Also, the effect of the tax audit on reported income for the top bracket is nine to seventeen times higher than for the lower income brackets. Thus, with limited audit resource, we recommend the government to push audit for the top income bracket.