This research presents a classification theory for the L2 vocabulary learning strategies. Based on the exploratory and confirmatory factor analyses of strategies that adult Chinese English learners used, this theory identifies six categories, four of which are related to the cognitive process in lexical acquisition and the other two are metacognitive and affective factors. Compared to other theories on language learning strategies, the uniqueness of this theory lies in that the cognitive factors correspond to the essential steps learners take in acquiring new words. This research does not support that a memory factor or a social factor exists independently in vocabulary learning strategies. (Contains 3 tables and 2 figures.)