In China, English as a foreign language (EFL) learning mainly occurs in the classroom, and the resultant lack of practice using English in authentic settings makes it quite difficult for many Chinese learners to learn English words. They may often feel that English words are "difficult to learn and easy to forget." As such, how to effectively teach English vocabulary in classrooms is an essential point for English teachers. Assuming that the goal of vocabulary teaching is to build up students' mental lexicon, this paper first briefly introduces the properties of the mental lexicon and examines differences between the mental lexicon and (print or electronic) dictionaries. More importantly, it then discusses how to teach English vocabulary based on a proper understanding of the organization of the mental lexicon and means of accessing it. Finally, the author poses some suggestions on vocabulary teaching: learning words from context as against word lists, establishing semantic relations between words, providing learners with frequent exposure to words, and teaching morphological knowledge pertaining to the words.