This study investigated, using pretest-posttest quasi-experimental research design, the Comparative effects of Igbo and English Languages as media of Instructions on academic achievement of Pupils in Public Primary Schools in Oshimili North Local Government Area of Delta State, Nigeria. The sample comprised 1,600 Primary 3 Pupils from six Public Primary Schools, randomly assigned to experimental and control groups. Four research questions and four null hypotheses guided the study. A reliable instrument, Comparative Assessment Test (CAT), was used to collect data. Difference in mean academic achievement and mean gain achievement were subjected to Z-test analysis and were significant in favour of those taught mathematics in Igbo. The implications and the recommendations of the study were advanced to improve teaching, learning and academic achievement of pupils in Public Primary Schools in Nigeria.
