This paper examines the function of mother tongue in second or foreign language teaching and learning, focusing on the case of Chinese learners of English. The use of L1 in L2 vocabulary teaching is studied by summarizing relevant theories and by analyzing an example of how Chinese teachers introduce new words to students. The influence of L1 in L2 vocabulary learning is explored through studying three major negative transfers of Chinese learners of English with abundant common and typical examples.Seven implications for L2 teachers are listed as the result of my study.