The purpose of this paper is to show that theoretical and empirical studies of agricultural trade should be conducted under the precondition that international markets are imperfectly competitive. The degree of imperfect competition in international wheat markets is calibrated as tariff equivalent. The results of the theoretical analysis showed that an erroneous assumption of perfect competition overestimated the effect of trade liberalization, while an increase in the degree of imperfect competition in international markets offset the benefit of trade liberalization. In addition, the results of the quantitative analysis showed that the present international wheat markets were imperfectly competitive, and the degree of imperfect competition had increased since the implementation of the Uruguay Round agricultural agreements.