ABSTRACT The purpose of this study is to investigate the influence of teachers’ limited English language proficiency to poor academic performance among secondary school students in Mwanza city. The objectives of the study were to investigate the causes of teachers limited English language proficiency, to find out the effects of teachers limited English language proficiency in academic performance, and to find out the possible solution to solving the problem of teachers limited English language proficiency among teachers in secondary schools in Mwanza city. The Teacher-student interaction theory, Gestalt Theory, and Total Quality Management Approach were used to help understanding how learning and teaching process depend much on the language of instruction. The study employed quantitative approach and descriptive survey research design. Questionnaires and structured interview guide were used to collect data. The population of the study included students, teachers, and school inspectors in Mwanza city. A simple random sampling technique was used and 300 students from ten schools, 90 teachers from nine schools and 7 school inspectors. The data gathered was analysed by using tables and figures. The results showed that the teachers limited English language proficiency is caused by a poor education system, whereby the system of education in Tanzania do not prepare students who will later be teachers to acquire enough English language skills, as a result, teachers lack English language proficiency. The results also showed that, poor English language teachers in schools are the cause students’ poor academic performance; hence teachers are not able to clarify concepts and so fail to transfer knowledge to learners. The admission of candidates has been also poor since most of the students selected to join the teaching profession are those who perform poorly in their academics. Generally, this causes poor academic performance because most of those who enter teaching professional cannot deliver the content in class. The issue of teacher training colleges and universities to empower student teacher also is a problem. Most of these educational institutions do not emphasize on the English language, though they are aware that it is a medium of instruction. The infrastructure and teacher availability in general is a problem in developing and empowering student teacher with good English knowledge, to assist their teaching skills in the classroom. The study recommends that the best students should be selected to join the teaching profession, but the government should first put attraction for people to join the profession. Universities and teachers' colleges should put emphasize on the English language to student teachers so as equip to them the basic English language skills which will assist them in content delivery to learners