The main policy goal of the corporate alternative minimum tax(AMT) is to enhance the horizontal equity of tax burdens among firms, which can be distorted by the excessive utilization of tax support programs. This study empirically analyzes the effect of the three AMT policy variables: the adjustment of the tax base, the AMT tax rates, and the designation of tax support system subject to the deduction limit under AMT, on the horizontal equity. Using 143,282 observations over the period of tax filing year 2009~2016, it is found that AMT deteriorates the horizontal equity inadvertently. The main reason is too low AMT tax rate for small and medium enterprises(SMEs) and too high AMT rates for the non-SMEs. The broader the scope of tax support programs that are subject to the deduction limit under AMT, the higher the Horizontal equity for SMEs will be. If policy makers are only interested in the additional tax revenues raised by AMT, they should consider a simple sur-tax that incurs lower compliance costs than AMT.