Korea and Japan rely on imports for most of the grain except rice, and the U.S. has maintained the highest shares of wheat, corn, and soybean import markets in both countries. This study aims to analyze the existence of asymmetric price transmission and market power that could occur in the U.S. grain export process targeting Korea and Japan. The results of asymmetric price transmission analysis using a nonlinear ARDL model show that, in the long run, when domestic prices (received by farmers) of wheat, corn, and soybeans in the U.S. increase, the impact on export prices to Korea and Japan is greater than when they decrease. Through the long-run asymmetry test, it was confirmed that there was statistically significant positive asymmetry in the domestic and export price transmissions of wheat, corn, and soybeans in the U.S. Also, in the short run, it was found that there is a positive asymmetry in the domestic and export prices of wheat and corn. Therefore, it is necessary to strengthen the monitoring of domestic and export prices of the U.S. to eliminate asymmetric price transmission. According to the result of the residual demand model estimation, residual demand elasticity was negative and statistically significant only in the case of corn and soybeans of Japan. In other words, it was found that the U.S. has market power in the corn and soybean import market of Japan, which is highly dependent on imports compared to Korea, but does not have market power in the Korean market. This suggests that, unlike Japan, in Korea's grain import market, there is a competitive structure between the U.S. and other exporting countries.