The current study examined the effect of an online professional development learning experience on teachers' self-efficacy through 148 (Male = 22; Female = 126) K-12 teachers and school educators. The Teachers' Self-Efficacy Scale (TSES) developed by Tschannen-Moran and Woolfolk Hoy (2001) was administered twice with a five-week gap. Additionally, all participants' descriptive self-analysis of their own score change was examined to analyze teachers' attributions of their self-efficacy change. Both quantitative and qualitative methodologies were used to analyze the data. The findings indicated that teacher efficacy increased as a result of their online professional development experience. Participants' self-analysis of their efficacy change provided some possible explanations for mixed reports for the influence of experience on teacher efficacy