The purpose of this paper is to determine teacher-related factors that affect academic performance of pupils on transition from standard three to standard four in public primary schools. The paper is guided by transition theory, which was developed by Schlossberg. The study was conducted at Lugari Sub-County. The study findings revealed that teacher related competencies like; qualification, experience, teaching methods and level of commitment influenced pupils' academic performance. The study recommends that more teachers need to undergo in-service training, teachers need to improve their relationship with pupils, government should employ more teachers, and curriculum supervisor should regularly monitor and evaluate teachers in schools.