Teachers' Professional Development Massive Open Online Courses (TPD-MOOCs) are a new form of MOOCs and have influenced an intense research interest. This study reports on the design and implementation of a TPD-MOOC which utilizes digital educational content and Open Educational Recourses (OER) and supports in-service elementary school teachers to enhance their self-efficacy beliefs. In the design framework we take into consideration the findings of previous research and the educational needs of the participants. We conducted an experimental design research and compared the teachers' self-efficacy beliefs before and after their participation in a TPD-MOOC. A total of 251 teachers enrolled in this course and 142 of them completed it. We used quantitative data to measure the enhancement of teachers' self-efficacy beliefs and the effectiveness of the course. The results provide evidence that our TPD-MOOC improved in service elementary school teachers' self-efficacy beliefs in teaching biological concepts. Recommendations are made for future research.