A growing number of studies have examined how second language learners acquire vocabulary with what sort of materials. Most of these studies, however, have only measured explicit memory of learners. Various taxonomies of memory have been employed to study human memory, and a series of research verified the existence of implicit memory. Implicit memory is a memory in which previous experiences help in the performance of a task without conscious awareness of these previous experiences, and explicit memory is a type of memory that is the conscious, intentional recollection of previous experiences and information. This article argues the importance of measuring implicit memory in vocabulary acquisition and provides some ideas to obtain accurate data at the implicit memory level in vocabulary learning.