Given principals' central role in school performance, this study seeks to explore factors that influence school principal competency. The study employed a quantitative research approach. A survey questionnaire was administered to 290 principals across six northwestern mountainous provinces in Vietnam. The questionnaire considered eight areas of principal competency, including self-development and the development of others; school organization and operation; staff development; instructional programme management; education quality assurance management; management of financial resources; engagement with families, community and government; and the school's strategic planning. Analyses of variance and correlation (Pearson's r) techniques were used, and the findings demonstrate that principals' competencies were statistically affected by both internal factors (gender, experience, health, family and IT skills) and external factors (school location, school demographics and school facilities). However, the study found no correlation between principals' academic qualifications and their competencies.