In general, it is known that the progressivity and income redistributive effect of income tax system show an inverted-U relationship. Using Household Income and Expenditure Survey data from 1990 to 2012, this research report empirically investigates the correlation between progressivity and income redistributive effect of Korea’s income tax system and gives policy implications. The result from empirical analysis is that the correlation between progressivity and income redistributive effect of Korea’s income tax system is inverted-U. Moreover, it supports the inverted-U relationship that the correlation between progressivity and income redistributive effect in change rate shows an negative relationship. This implies that the ‘law of diminishing marginal income redistributive effect’ works when the progressivity of income tax system increases by a unit. That is, the income redistributive effect increases at first but decreases after a highest value as the progressivity of income tax system increase. In 2012, the progressivity and income redistributive effect of income tax system as the Kakwani index of 0.3146 and the income redistributive effect of 1.3247 is on the right-hand side of the inverted-U correlation graph. Therefore, it concludes that Korea’s current income tax system already has an higher progressivity so that the income redistributive effect gets lower. If the present government raises the progressivity of income tax system, to expand the tax revenue and realize the equity in taxation, then the income redistributive effect could be weaken as a result deviated from the policy intended.