Language of instruction in education system in Tanzania is an issue which has been drawing sporadic debates among education stakeholders and ordinary people. Some of the critics urge that language of instruction plays a key factor in determining the overall academic performance of secondary school students. This study was conducted with the objective of examining whether English as the language of instruction contributes to academic performance of students. The researcher employed questionnaire and tests to obtained data. 16 teacher respondents filled in questionnaires whereas 375 students in experimental and control groups did Kiswahili and English tests respectively. Those tests were the same except that experimental group did a test in Kiswahili while control group test was administered in English. The findings showed that language of instruction may contribute to students‟ academic performance. It was revealed also by findings that the more the student is comfortable in using the chosen language of instruction the better; the better is his/her performance when taught well in that language. The overall results showed that students in experimental group performed better than their counterpart in English group. In order to transmit knowledge and skills which are vital ingredients for individual and social development students as well as teachers need to be conversant with the language of instruction. It is high time the government switched to Kiswahili as the language of instruction in secondary schools because the current English language of instruction is a hindrance to successful learning.