the present study first discusses the importance of vocabulary in EFL learning and teaching, and the necessity to carry out explorations into Chinese EFL learners' English mental lexicon. It aims to find out the formational and developmental features of Chinese EFL learners' English mental lexicon as well as the factors that influence the formation and development of the mental lexicon. Based on the findings of the present study, suggestions for EFL vocabulary learning and teaching and recommendations for future studies are put forward.