The main purpose of this study was to determine the effect of clinical supervision of principals on teachers' teaching performance in secondary schools. A total of 100 public secondary school teachers from Kuala Selangor, Malaysia participated in this study and data was collected through distribution of a survey questionnaire. Both descriptive and inferential statistics were used to analyse the collected data by using SPSS software. Results revealed that effective clinical supervision process will consist of steps of pre-observation planning, observation implementation and post-observation monitoring. Moreover, results from teachers' perceptions proved that clinical supervision positively influence the teaching effectiveness in terms of writing of daily lesson plan, set induction, development and delivery of lessons, questioning techniques, student involvement, encouragement, training and student assignments, examinations, assignments and exercises, closure of teaching and classroom management. However, in this study, the level of clinical supervision based on the pre-observation planning practiced by principal resulted high.