This paper reports on the study of private theories of Science, Technology, Engineering and Technology ("STEM") teachers and their learning design practices at International Schools in Hong Kong. The literature emphasises that teachers are the key to successfully implementing STEM education. However, most are unprepared and ill-equipped for the task of preparing students for a sound STEM education. Teachers' thinking is critical for their learning design, decision-making and implementation of STEM. This decision-making is underpinned by a set of private theories. Using the methodology of a qualitative multi-case study, this research focuses on five participants teaching STEM in International Schools in Hong Kong. The private theories of the cases were identified and observed for any change with the use of a learning design model for STEM education. The learning design is introduced using a novel methodology of intervention. An observable change occurs in the majority of cases after the intervention, however certain private theories remained an obstacle to the effective implementation of STEM education. To overcome the remaining private theories in effective implementation of STEM education, the study proposes a novel framework incorporating both learning design and collaboration to mediate teachers' thinking in the context of STEM learning design.