This study aims to examine the efficacy of Mobile-Assisted Language Learning (MALL) of English as a foreign or second language (L2) through two perspectives: learning gain and learner autonomy. Previous studies have shown that L2 learning combined with media could activate the learning processes, resulting in an easier recall of the target vocabulary required in L2. In addition, mobileassisted L2 learning could also enhance autonomous learning inasmuch as successful MALL would have to rely mainly on the autonomous learner even in learning contexts where the goal and task are already fixed. Based on this standpoint, the study hypothesizes that the engagement in L2 learning with mobile devices along with a classroom-based writing course could make L2 learners not only achieve the target L2 lexis effectively, leading to better L2 writing performance, but also help them to be more autonomous even in a setting when the task and goal are fixed. To test this hypothesis, both empirical and questionnaire studies were conducted for Japanese undergraduates (n=94). Based on the results of three weeks of L2 academic writing practice between groups learning with and without mobile devices, the findings of our t-test analyses of learners' vocabulary recall and a questionnaire survey about learner autonomy suggested that MALL significantly contributed not only to L2 vocabulary recall in comprehensive and productive tests, but also to enhancing positive attitudes towards autonomous learning.