The study investigated the effects of assignment teaching method (ATM) on academic achievement of students in business studies in Oyo State, Nigeria. Two research questions guided the study and 1 hypotheses were tested at 0.05 level of significance. The population for the study was all junior secondary school business studies students in Oyo State, Nigeria. Purposive sampling technique was used to select a sample of 93 students for the study. Pre-test, post-test non-equivalent groups quasi-experimental design was used for the study and the instrument for data collection were programmed book in business studies and business studies achievement test (BSAT) which were developed by the researchers and validated by two experts. A reliability co-efficient of 0.72 was obtained for the test items using split-half reliability technique. Mean was used to analyse data relating to research questions while analysis of covariance (ANCOVA) was used to test the hypotheses. Findings revealed that students taught business studies using ATM achieved better with higher post-test mean score than those taught using conventional teaching method. Findings revealed that no interaction effects of teaching methods and gender on students’ academic achievement in business studies. It was recommended that business studies teachers should use assignment method in teaching so as to enhance students’ academic achievement in the subject.