This study examines the tax audit impact on voluntary compliance. It is different from those in the literature in several ways. First, models were built exclusively for investigating the voluntary compliance behavior shifts after a firm is audited. Second, apart from the theoretical approach and laboratory experiment approach used in the literature, this study applied the difference-in-differences non-experimental approach. Third, historical population data of a New York State economic sector were used in this study instead of experimental data or randomly selected sample data often used in the literature. The results of both Ordinary Least Squares (OLS) and Time Series Cross Section (TSCS) autoregressive modeling methods are presented. The results of both methods suggest that after an audit, a firm would report a higher sales growth rate. The TSCS approach shows that in the year of the audit, a typical firm would report a sales growth rate which is 2.63 percentage points higher than a firm that was not audited. The percentage would decline by a rate of 1/3 each year thereafter. The findings suggest that the audit productivity derived in many research papers, where only the direct audit collections are considered, may be underestimated. The results of this research may provide policy makers with extra incentives to strengthening the audit efforts to generate more revenues.