A principal's supervision competency affects the improvement in the teaching and learning process; therefore, it needs to be improved. This study aimed to describe the principal working group (PWG) activities and the principal's supervision competencies after being assisted by school supervisor in the PWG at Sangkuang Island, East Kutai Regency. This research used a qualitative method and was conducted at state elementary schools. Data collection used interview, observation and documentation techniques. The data were analysed using Miles and Huberman's model. The results showed that the PWG activities were effective and the principals could exchange ideas and solutions to solve school problems. The school supervisor's assistance in the PWG activities increased the principal's supervision competencies. The principals were able to plan supervision programmes properly, conduct standard supervision and implement follow-up supervision according to the needs. This study implies that a school supervisor's assistance is important to improve the principal's supervision competence and this can be adopted at other area.