Formative assessment remains one of the most regular assessment tools which most teachers adopt in the process of assessing the performance of students. However, apart from just providing insight to the teacher, these methods over the years remain controversial on its ability to help students in gaining mastery of course content. Hence, the study focus on effect of formative assessment methods on mastery of mathematics course content by junior secondary school students in Obio-Akpor LGA of Rivers state. The study adopted the pretest-posttest quasi-experimental research design. Three specific objectives and corresponding hypotheses guided the study. The population of the study consisted of 14,350 junior secondary school students in public schools in Obio-Akpor LGA of Rivers State. The sample size included eighty (80) participant drawn using multi-stage sampling process. The “Junior Secondary Mathematics Test” (JSMT) was used as instrument for data collection. Table of specification was used to validate the test while KR21 was used in determining a reliability index of 0.88. Participants were randomized into three experimental groups and one control group. Specifically, group A, B and C consisted of 20 participants who were given class test, take home assignments as well as engaging in class discussions respectively. Group D was tagged as the control group. In determining the cut off score for mastery of JSMT, the Angolf method was used by sending JSMT to five judges. From the average, the overall percentage for the five judges stood at 67.7% indicating that for any students to be considered and mastering the “Junior Secondary Mathematics Test” (JSMT), it is required that 34 of 50 test items must be correct. Mean, standard deviation and independent t-test was used in data analysis.