This study investigated the effect of motivational factors on English achievement in an intensive English course. The participants consisted of 41 Iranian male military staff, with the age range from 21 to 28. The participants filled a translated and adapted version of the mini-Attiude Motivation Test Battery (a=0.70). Factor analysis of the questionnaire yielded four principal variables namely, motivation, integrativeness, organizational influence, and anxiety. Descriptive statistics indicated that the military staff were highly motivated in learning English and had low English learning Anxiety. It also suggested that the military organization was not so much supportive in learners' studying English. Furthermore, path analysis indicated that integrativeness predicted the motivation to learn English positively, and that motivation was a positive predictor of English achievement, whereas organizational influence was a negative predictor of English achievement. This study confirmed that motivation is the single most influential factor of language learning achievement, all other things being equal. Finally, based on the current study a path analytic model of L2 motivation was proposed.