This study explores the impact of a STEM integration teacher professional development program focusing on teachers’ perception of engineering and their attitudes toward integrating engineering into teaching. A total of sixty-eight teachers from ten schools participated in the program for five days. Data are collected from three main sources including (1) pre and post concept maps probing teachers’ perceptions about the engineering discipline, (2) a pre and post survey measuring teachers’ self-efficacy of teaching science/mathematics within the engineering context, and (3) engineering integrated science and (or) mathematics lesson plans and teaching reflections. This study utilizes both qualitative and quantitative research methods depending on the data we have collected. The results show that both science and math teachers thought that integrating engineering into teaching provided valuable outcomes, i.e., promoting students’ learning about engineering and improving their interest in science or math through real-world problem solving exercises. Participants also felt more comfortable about integrating engineering in their teaching after the program. The results also imply that the teachers’ understandings of engineering become more concrete after the program. This study also provides an overview of the challenges and advantages of teaching engineering in K-12 science and mathematics classrooms.