This study was undertaken for the purpose of investigating ‘’the effectiveness of Management By Objective approach as a tool of improving performance in Ilemela district secondary schools in Tanzania’’. Specifically, the study intended to achieve the following objectives: to find out the extent to which education managers involve their subordinates (teachers) in policy planning and decision making, to investigate the mechanism put in place to ensure proper implementation of the MBO approach in secondary schools, to examine the challenges faced while implementing the MBO approach in secondary schools, and to examine the suggestions towards proper implementation of the MBO approach in secondary schools. The study was conducted in Ilemela District in Mwanza Region. A total number of 127 respondents was covered including 112 teachers, 14 Heads of schools, and one District Education Officer all of whom were randomly and purposively sampled. Interview, documentary review and questionnaire were the instruments used in data collection. The findings were analyzed using thematic and content analysis techniques. Findings reveal that teachers are not involved in decision making and policies formulation because most of directives, policies and decisions are made from the Central level to be implemented by teachers in secondary schools. Hence this affects effectiveness of the MBO approach. Staff meetings, school management team, academic department meetings and school board meetings are the mechanisms put in place to ensure the effectiveness of MBO approach in secondary schools. Budgetary constraints, pressure of time, the transparency, communication and leadership support are the challenges affecting the effectiveness of the approach in secondary schools. Also the study reveals different suggestions for effectiveness of the approach such as good working environment, rewards and motivation to teachers, recruitment of qualified personnel in teaching profession, and formulation of strict laws and regulations to increase accountability