In this paper, I specifically focused on teacher’s belief system that impacted the course development. Utilizing the methodology called Formative and Design Experiments (FDE), a Japanese language teacher and I collaboratively designed and implemented interventions to create better learning environments for their students. The FDE procedure enabled the teacher to reflect upon her beliefs for online instruction and scrutinize the reason why the interventions worked. The results showed the change in the teacher’s belief system and understanding of online Japanese language learning during the course of the research, which led to further revisions and implementations. As a result, this research contributed to improving the online Japanese language course, demonstrating how the teacher’s beliefs in online L2 teaching have changed and how the change influenced her to refine her delivery and management of the course.