This study identified the nature of principals' supervisory roles and the perceived effectiveness of principals in the supervision of teachers' instructional tasks. Furthermore, it investigated the constraints faced by principals in the performance of supervisory duties in the teaching-learning process. This was with a view to providing information on the utilisation of principals' roles in enhancing quality assurance in secondary schools. The study employed the descriptive survey design. The target population comprised principals and teachers in secondary schools in Ondo state. The sample consisted of 60 principals and 540 teachers randomly selected from 60 secondary schools. The secondary schools were selected using stratified random sampling method from 5 Local Government Area [LGAs]. Three research instruments were used for data collection; they are Principals' Supervision Rating Scale (PSRS), Interview Guide for Principals (IGP) and Teachers' Focus Group Discussion Guide (FGDG). Three research questions were resolved based on percentage and mean scores. The results showed that most principals accorded desired attention to monitoring of teachers' attendance, preparation of lesson notes and adequacy of diaries of work while tasks such as the provision of instructional materials, reference books, feedback and review of activities with stakeholders were least performed by many principals in secondary schools. The study concluded that challenges that principals faced in the tasks of institutional governance, resource inputs, curriculum delivery and students' learning require effective collaboration and goal-oriented synergetic interrelationship between the school and the relevant stakeholders in its environment.