This paper aims at studying how the Japanese learners of English have been influenced by their mother tongue by means of analyzing their written language from the perspective of grammatical categories．Data was collected from the compositions written by the second-year students of English Department at a university．These students had studied English in Japan for about six years before they came to China．Errors found in their essays were thus analyzed，mainly in the six grammatical categories of number，gender，case，person，tense，and word order．Through the elaboration on the differences of the two language types between English and Japanese，this paper attempts to raise EFL teachers' consciousness of interlanguage development of EFL learners in order to better understand the features of the errors committed by EFL learners and to facilitate effective language teaching．The result of the study also provides further materials of language facts for theoretical research in linguistics．