Mortgage subsidy, particularly in mortgage interest deduction, has been a controversial topic that fewer and fewer debates were willing to touch on. As the third largest component of government tax expenditure, evaluating mortgage subsidy is of great importance when evaluating how effectively the government budget has provided households with better financial well-being. This paper discussed the opponent condition of mortgage interest deduction and its resulted excess burden. Given the subsidy shows vast heterogeneity across income groups and geography due to itemization heterogeneity, I proposed the theoretical explanation by explicitly including unit user cost of housing consumption per the difference in tax deduction choice. I further conducted an itemization share analysis to support my proposition about the relationship between mortgage loan characteristics and itemization decisions. Moreover, I calculated and contrasted the excess burden of itemization carried across income groups and loan-to-value ratio thresholds. I found households with a low loan-to-value ratio are less likely to itemize their tax deduction for receiving the subsidy, and households facing higher mortgage interest rates are more willing to itemize. The excess burden is also distributed unevenly across income groups, same as how the benefits are disproportionately received.