This study investigated the intervention of a weekly learning diary on design students' self-regulated learning in an online learning environment. A total of 54 undergraduate students from an intermediate and an advanced course respectively participated in this study. In a 7-week period of online learning, the students had to complete a course project on their own. Synchronous online learning with the Microsoft Teams program was adopted for students to have real-time learning as well as recordings for after-class reviewing. The results revealed that the structured weekly diary was helpful for the students' goal setting, time management, self-monitoring, and self-evaluation. Students with clear strategies of time management mostly had better performance. The students had an upward trend of self-monitoring and self-evaluation. Regarding their self-efficacy, the freshman class did not change greatly over time but the senior class steadily built up confidence in online learning and self-regulated learning. Nevertheless, the process and experiences of online self-regulated learning with the intervention of the learning diary influenced the design students to a certain degree.