The current study investigated the application and effectiveness of computer assisted language learning (CALL) in teaching academic writing to Iranian EFL (English as a Foreign Language) learners by means of Microsoft Word Office. To this end, 44 sophomore intermediate university students majoring in English Language and Literature at an Iranian university who had enrolled in a course called Advanced Writing were randomly divided into two groups. As a pre-test, a pen-and-paper writing task was given to both groups at the beginning of the semester. The control group including 24 male and female students was taught based on traditional approach while the experimental group including 20 male and female students was taught based on CALL. At the end of one semester, a pen-and-paper writing task was given to both groups. The results of the post-test revealed that the students who were exposed to computer-based instruction outperformed their counterparts in terms of using appropriate articles, tense, plural forms and spelling. Moreover, the students in the experimental group produced paragraphs of higher quality. The findings of this study confirmed the efficacy of computer-based instruction in the development of EFL learners' writing skills.