One current policy in Thai government education is to enhance teachers' understanding of Science, Technology, Engineering and Math (STEM) educational techniques at every level. This study reports the results of a professional development program to assist 134 in-service teachers from 23 small primary schools in rural of Lopburi Province to conceptualise concepts of STEM education and learn how to implement it in their own contexts. Results show that after participating in a two-day workshop, the teachers' understanding had increased and they appeared to value the advantages of STEM education methods. However, generally, only teachers with science backgrounds were effective in implementing STEM learning activities to help develop student thinking skills and creativity.