Due to the complexity of the alternative minimum tax (AMT), marginal tax rates (MTR) estimation studies deem AMT firms’ MTRs almost impossible to estimate. In this paper, we develop an analytical model for estimating AMT firms’ MTRs based upon the explicit and implicit marginal tax costs of the AMT. Our model shows that AMT firms’ MTRs are essentially the opportunity cost of AMT payments. We re-examine two prior empirical studies in terms of AMT MTRs instead of traditional MTRs, and we found that our result is much better than previous studies. In addition, we also found that the impact of applying AMT MTR in empirical estimates may depend on the proportion and characteristics of AMT firms in the sample.