This study aimed to determine the effect of in-service training programs on the professional development of public elementary school teachers in Candelaria West District, Division of Quezon. The study used quantitative and qualitative research designs and applied a descriptive-correlational method. The results revealed that professional development in terms of content knowledge and pedagogy, learning environment, diversity of learners, curriculum and planning, assessment and reporting, community linkages and professional engagement, professional growth, and professional development are positively, highly, and significantly related to the in-service training programs' professionalism dimensions and pedagogical components. Likewise, the findings showed that teachers had significant improvements in content knowledge and its application within and across curriculum areas; strategies for promoting literacy and numeracy; strategies for developing critical and creative thinking as well as higher-order thinking skills; teaching and learning resources, including ICT; and learners' gender, needs, strengths, interests, and experiences after attending a number of INSETs. The result suggests teachers should attend INSET programs with a positive attitude and active participation to improve their knowledge, develop their skills, change their behavior and attitudes, and have the ability to perform tasks more efficiently and effectively.