This study focuses on investigation of district school quality assurance officers’ (DSQAOs) understanding of competence-based curriculum (CBC) and how they conduct CBC quality assurance in secondary schools in Tanzania. The study employed interpretive paradigm, qualitative approach, a case study design, and was guided by Bernstein's theory of the pedagogic device. Forty (40) respondents were purposively and conveniently selected including DSQAOs (n = 12), head of schools (n = 4), and teachers (n = 24). Data generation methods were semi-structured interviews, participant-assisted drawings, and documentary review; while content and thematic analyses were employed. Findings revealed that DSQAOs had varied understandings of CBC. Some understood CBC as a curriculum that supported students’ studies and learning environment. Others understood it as a curriculum that supported students’ acquisition of competences needed in their daily life; while others failed to explain its meaning. Moreover, DSQAOs conducted classroom observations and gave feedbacks for improving teaching and learning activities in schools. However, DSQAOs’ use of harsh language to teachers, limited time during school visit, inadequate report writing, and lack of clear conceptualization of CBC concepts hindered effective support to teachers. The study concludes that in order to conduct effective CBC quality assurance in schools, selection of competent DSQAOs and regular trainings are imperative.