ABSTRACT
The study investigated the effect of formative Assessment on students’ achievement in secondary school Mathematics. Three hypotheses guided the study. The experimental research design was employed.. One hundred and twenty (120) Mathematics students in secondary II Art classes in two public schools in Iseyin Local Government of Oyo State, Nigeria selected through purposive technique made up the study sample.. Formative Test I, II and III and Mathematics Achievement Test (MAT) were used for data collection. Data were analysed using paired sample t-test and independent sample t-test statistical tools. Findings from analysis revealed that formative assessment has a strong significant difference in the mean achievement score of Mathematics students that are exposed to it (t = 36.54, p =000) while there is no significant difference in the mean achievement scores of student who are not expose to formative assessment (t=2.053, p = 0.045). Also, there is no gender difference in the achievement scores of Mathematics students that are exposed to formative assessment (t=0.112, p = 0.053). The study recommended that all School Administrators should emphasis the use of formative assessment by all teachers and they should allow, encourage and provide incentives for them to attend seminars, workshops, conference and in-services training to enhance their performance and to acquire necessary skills to constructing formative tests.