The present study investigated the effects of a self-monitoring strategy on web-based language learning. Both students' academic performance and their motivational beliefs were investigated. The interaction between the use of a self-monitoring strategy and the level of learners' English proficiency was also examined. A total of 99 college students who were enrolled in classes for Freshmen English participated in this study. The experimental group was led to a web page with self-monitoring form for recording study time and environment, learning process, predicting test scores, and self-evaluation while the control group was not. It was found that (1) the self-monitoring strategy had a significant main effect on students' academic performance and their motivational beliefs; students who applied the self-monitoring strategy outperformed students who did not apply the self-monitoring strategy on both academic performance and motivational beliefs regardless of their English proficiency level; and (2) the influence of self-monitoring was greater on the lower English level students than on the higher English level students. The positive findings suggest that encouraging students to develop self-monitoring could help increase the success of online learning. Thus, applying a self-monitoring strategy is strongly recommended for web-based instruction.