Recently, the translation quality of machine translation has been drastically improved. While not yet perfect, compared to the writing of a typical foreign language learner, machine translation exhibits few grammatical errors, even considering the context of the entire paragraph. This study is an attempt to analyze the effect of machine translation on English writing instruction. In the English writing class for this study, students were instructed to take advantage of machine translation to engage in free writing activities. And ‘writing efficacy questionnaire’ and ‘free English writing’ were conducted at the beginning of the semester and at the end of the semester. The English writing instruction using machine translation were analyzed to have paid off to great results: despite the students’ limited English proficiency, the writing efficacy of all the students showed a significant increase. In general, the upper class students showed greater increase, and the lower students’ increase were relatively limited. However, regardless of the differences in the students’ English proficiency, the amount of English composition by every student has increased. Therefore the use of machine translation in English writing classes is considered to have achieved the desired results.