The goal of this paper is to examine Thomas Aquinas’s thought on economic justice based on his theory of the just price and usury. This paper insists that his theory of the just price and that of usury are intrinsically connected by sharing one common moral precept that nobody may not sell what he does not have. Aquinas’s double principle for determining just price leads to his distinction between damnum emergens and lucrum cessans in his theory of usury. Aquinas approved the claim on damnum emergens whereas he negated the claim on lucrum cessans. Aquinas’s treatments of damnum emergens and lucrum cessans shows that he was well aware of the tension between the doctrine of moral theology and the reality of economic life and that he attempted to reconcile this tension.