Dissertation (MA Education)The purpose of the study was to explore the influence of English language as a medium of instructions on students’ Mathematics understanding. It employed a descriptive research design using qualitative and some aspects of quantitative research approaches. A total of 131 respondents were included in the study. These were heads of mathematics department, Mathematics teachers and form IV students. Purposive and random sampling techniques were used to obtain respondents of the study. Purposive sampling was used to obtain heads of mathematics department and form IV students and simple random sampling was used to obtain schools and Mathematics teachers. This study was guided by cognitive Theory of Jean Piaget. Questionnaires, interviews, and observation were used to collect the data were employed in the study in attaining goals and providing the necessary direction to ensure the compatibility with the overall objectives of the study. Qualitative data were analyzed according to their pattern of descriptions from the respondents to generate themes and sub-themes while quantitative data were processed and analyzed using SPSS (version 20) to compute frequency percentages of responses, mean average and overall mean through descriptive statistics. The study found that most students had insufficient English language mastery due to their background of their previous education level. This led to poor understanding, dropout, failure to speak and write and low grade achievement. As a result, it was found that teachers use code switching, code mixing and translation strategies to enable students understand mathematics. The study recommends that further research should be done on other subjects to make rational decision about the language of instructions.