In the 1939 Battle of Khalkhin Gol against the USSR in Mongolia, the Ki-27 faced both Polikarpov I-15 biplane and Polikarpov I-16 monoplane fighters. In the initial phase of the conflict, its performance was a match for the early model I-16s, and was considerably superior to the I-15 biplane. With better trained Ki-27 pilots, the IJAAF gained aerial superiority. The Ki-27 was armed with two 7.7 mm (.303 in) Type 89 machine guns and as with most aircraft of the period, lacked armor protection for the pilot and self-sealing or fire suppression in the fuel tanks.