This study focused on assessment of teachers` perceptions on “Big Results Now” (BRN) implementation in Tanzanian Ordinary Secondary Schools in Ukerewe District. The study was guided by the theory of change. The study involved 105 participants from 10 government Ordinary Secondary Schools from Ukerewe District. The study used mixed approach particularly explanatory mixed design in which data collection instruments were questionnaire and interview. The quantitative data were analyzed using descriptive statistics with the help of Statistical Package for Social sciences (SPSS) version 20 whereas qualitative data were analyzed using thematic analysis. The quantitative data were presented in frequency, percentages and tables while qualitative data were presented in themes. The main findings of the study showed that the opinions from teachers proposed to involve all subjects in all classes during the preparation of BRN program in Ordinary Secondary Schools. The BRN program needs modification for better implementation. Moreover, the government has tried to improve some of the initiatives such as a provision of capital grants, provision of teaching and learning materials and constructions of science labs in Ordinary Secondary Schools. The study recommends that involvement of teachers on BRN program is vital for the effects of its implementation.