Alternative minimum tax (AMT) is supposed to guarantee the corporate tax burden equity (“equity” hereafter), limiting the whole tax benefit of a firm and thus forcing the firm to pay the rock-bottom tax burden liability proportionate to its economic ability. We tested empirically whether the AMT actually improved the equity as a whole and in each segment of corporate groups, and whether the equity results might differ before and after 2004 when both basic corporate tax rate and the AMT rate for large firms stayed unchanged while the AMT rate for small-to-mid sized firms were lowered. Under the current corporate tax regime, virtually single-rate taxation, the equity can be measured by either how small the variation of tax burden rate is or how well the distribution of tax burden liability is proportionate to income. We used both the coefficient of variation (CV) of corporate tax burden rate and the coefficient of residual variation (CRV) of corporate tax burden liability to measure the equity. We estimated the regression line with no intercept in calculating the CRV to specifically emphasize the single-rate taxation property. We randomly selected 405 sample firms headquartered in Seoul which have reported positive annual net income consecutively since 2001. While the prior studies mainly analyzed the equity of individual income tax which has a significant progressive tax rate structure, this study focused on the corporate income tax which has a single tax rate structure and, thus, didn’t have to divide the sample firms into ability-based subgroups. We found following empirical results: First, the AMT improved the overall equity during the sample period, in 2003 and 2004. Second, the improvement of the equity among large firms were higher than that among small-to-mid sized firms in 2003 and 2004. The equity is construed as proportionate to taxpayer’s probability to pay AMT. We expect the contrary result since there are lots of tax benefit provisions in the current corporate income tax law in favor of small-to-mid sized firms and thus they are more likely to pay AMT. We presume that our finding is due to either the fact that the large firms, which have a high level of tax law knowledge, used the tax benefit provisions more aggressively or the fact that the small-to-mid sized firms in this study were not able to utilize many tax benefit provisions as they are located in a metropolitan Seoul area. Third, in 2003 and 2004, the AMT improved the equity among primary-and-secondary industry firms more than that among trinary industry firms, strengthening the single-rate taxation property more in the primary-and-secondary industry firms group. We expected the primary-and-secondary industry firms were more likely to pay AMT as there are more tax benefit provisions in favor of them. Our finding confirms the expectation. Last, the AMT worsened the equity of small-to-mid sized firm from 2003 to 2004 relatively more compared to that of large firms. We expected that small-to-mid sized firms were relatively less likely to pay AMT in 2004 than in 2003 since both basic corporate tax rate and the AMT rate for large firms stayed unchanged while the AMT rate for small-to-mid sized firms were lowered in 2004. Our finding confirms the expectation. This study is significant in that it verifies whether the AMT provision has achieved the intended economic function, using plentiful and timely sample firms data and analyzing the recent effect on the equity caused by the asymmetric change in the corporate tax rate scheme. Compared to prior studies, we showed that the equity is proportionate to the probability to pay AMT. We also showed that the asymmetric change in the corporate tax rate structure affects the probability to pay AMT, resulting in the shift of equity.