Wheter ability grouping is better than random grouping has been researched and controversial for many years. This paper tried to investigate learners' achievement motivation in learning English as a foreign language at two military academies. A total of 161 cadets at Fu Hsing Kang College (FHK) and National Defense Management College (NDMC) were asked to answer the questionnaire evaluating their achievement motivation in learning English. The cadets at FHK were grouped into two grades-the ability and the normal, while the cadets in NDMC were grouped into four grades according to their English ability. To compare the difference of cadets' achievement motivation among those groups is another subject of this paper. The results show that: 1.The achievement motivation of cadets in the ability groups is higher than that of normal groups at both academies. 2.There exists no significant difference in achievement motivation between ability-grouped and normally (randomly)-grouped studped students who were below the super lever in English at these two academies. 3.Several suggestions are given for practical application in teaching English at the military academies.