School Principals' competencies could be improved through continuous professional development (CPD) programme. Such a programme for principals in Indonesia has been implemented through direct modality since 2014. However, the implementation of this programme has lacked evaluation. This research aims to (1) assess the supporting and inhibiting aspects that contribute to the programme's success, and (2) measure the effectiveness of CPD in improving the school principals' competencies. This research was a survey involving 46 school principals in three Districts of Central Java, Indonesia. Both quantitative and qualitative methods were employed in this research. Questionnaires and interviews were used to collect data, which were analysed using a descriptive quantitative method supported by data source triangulation. The research shows that (1) the training process has been implemented with appropriate components, including assessment, learning materials, and delivery modes. Nevertheless, training facilities, infrastructure, and locations are less than conducive to learning, and (2) CPD does not fully contribute to improving principals' competencies, human resource management, and quality of instruction.