The focus of this study was to determine the relationship between the use of English as a language of instruction in public secondary schools and its effect in students’ learning. The objectives were to explore the outcomes of using English as a language of instruction in secondary schools, to find out the challenges teachers and students face in using English as a language of instruction in learning process as well as to identify the possible ways to improve the use of English as a language of instruction in public secondary schools. The study was conducted in Misungwi district in Mwanza region. The study collected data from students, teachers and heads of schools through questionnaire, interview and observation. The data were analyzed descriptively as well as statistically. The study found that many teachers and students are not fluent in the use of English as the language of instruction, they have been using two languages in teaching and learning process, that is Kiswahili and English. This is due to the fact that Kiswahili has been their language of instruction at primary level, when they reach at secondary level, they find English language as the language of instruction, therefore students fail to understand the subject content in English which leads to poor academic performance. The study concluded that English language should be used as a language of instruction from primary level to secondary level; this will help the students to be proficient in using it. The study recommended that the Ministry responsible with Education in Tanzania has to emphasize on the use of English language in secondary schools effectively as a language of instruction at all levels. The Education language policy makers have to think the proper ways on the use of English language as a language of instruction from primary to secondary level. Also, it can be recommended that classroom teachers have to avoid using two languages in learning process. Moreover, parents should support their children in learning English language making sure they create conducive environment of learning English as a language of instruction example buying English books for private study of their children after school.