Professional development is a vital process enhancing in-service teachers' professional qualities. This case study investigated an in-service teacher's possible selves and factors influencing his professional development. The participant was a project-based teacher in a constructionism school in Bangkok, Thailand. He was selected as the subject for this study due to the rich data he shared with the researchers in a larger-scale research study. Semi-structured interviews and personal diaries were the main instruments employed to reveal his possible selves, goals, and investment in the professional developmental process. An interplay of possible selves and the factors involved were analyzed and presented using narrative inquiry techniques. The findings revealed that the teacher invested heavily in the professional development process to improve his project-based learning pedagogy and English proficiency. The congruence of his ideal and ought-to selves, his clear goal, and his positive attitudes toward professional development were clearly observed. This clearly influenced his investment in professional development. Implications and suggestions for the teacher's professional development and future research were discussed.