According to Genesis 3, the ancestors of human race committed original sin by breaking God's precepts. Due to original sin, human nature is separated into two states: the innocent state before committing original sin and the fallen state after committing original sin. The difference in between is mostly that the former state is bestowed with God's transcendental gift-original justice. "And God saw every thing that he had made, and, behold, it was very good" (Genesis 1: 31). "For every creature of God is good," as Paul the Apostle pointed out (Timothy 4: 4), indicates that human race is made to be upright; everything about men is made to be good, and there should be nothing sinful. God made human beings inclined toward doing good "God hath made man upright; but they have sought out many inventions" (Ecclesiastes 7: 29). This very uprightness refers to God's transcendental gift to the ancestors of the human race, original justice, which inclines human race toward doing good and being with God as well as keeps them away from sufferings and death. The concept of original justice is actually associated with original sin, and its immanence is essential in explaining the fallen state of human nature. This article analyzes Aquinas's explanation about the concept of original justice to reach two goals: first, to point out the differences between the states of human nature before and after original sin by explicating the content of original justice; second, through differentiating "original justice" from "human nature" to explain why our forefathers, even in the innocent state, could still commit original sin.