Learner autonomy is believed to play a leading role in life-long learning; as a result, a myriad of studies have been conducted to discover tools to promote learner autonomy (e.g., blogs, Facebook, web 2.0, learning contracts, etc.). Nevertheless, the effects of a portfolio on promoting learner autonomy have not been explored much. This paper endeavored to investigate whether or not learner autonomy was developed with the implementation of the portfolio-based learner autonomy development (PLAD) model in an EFL writing course. Regarding research methodology, both qualitative and quantitative data were collected from a questionnaire with closed-ended and open-ended items and reflection logs kept in portfolios. For data analysis, the quantitative data were analyzed through descriptive statistics and inferential statistics, whereas qualitative data were analyzed through content analysis. The findings showed the development of learner autonomy after the course in which a portfolio was employed as a learning tool. In particular, the participants were aware of learner autonomy and felt positive about autonomous learning tasks, especially writing reflection. Self-assessment, however, was the most challenging task which they encountered because of lack of confidence and experience. In addition, it was evident that the participants could choose learning materials when they were offered freedom and rights to do this task.