The effective implementation of STEM education relies on qualified and well-educated teachers who employ good practices for integrated STEM education. Quality STEM education equips children with the necessary skills to address 21st-century challenges, playing a critical role in developing critical thinking, communication, collaboration, creativity, problem-solving, and responsibility. In this context, preschool and primary school teachers were recommended to attend STEM Professional Development (PD). This study aimed to determine the impact of the EarlySTEM Leader Teacher PD Program on teachers' ability to design STEM lesson plans, using mixed method. Researchers analyzed the STEM lesson plans prepared by preschool and primary school teachers (n=33) who participated in the integrated STEM PD program. All participants who met the graduation criteria were included in the study. The plans were evaluated using the Authentic Problems of Knowledge Society (APoKS) rubric. The findings showed that the Integrated STEM PD program positively influenced teachers' lesson planning skills, specifically when writing the APoKS. The implications of the study were discussed in detail.