Abstract: The study examined the mode of assessment utilized by Social Studies teachers in Junior Secondary Schools in Osun State. It investigated the students’ achievement in Social Studies and as well examined the influence of variables such as teachers’ qualification and gender on the modes of assessment adopted by the teachers in the study areas. These were with the view of providing information on how students’ achievement in Social Studies could better be improved upon in Osun State Nigeria. The study employed survey research design. The sample size consisted of 800 Social Studies students and 40 Social Studies teachers selected using multistage sampling technique. Two instruments were used for the study: Students’ Social Studies Achievement Records (SSSAR) and Teachers’ Modes of Assessment Inventory (TMAI). Data were analyzed using frequency counts, simple percentages and Chi-square statistics. The results among others, showed that majority of the teachers 27(67.5%) agreed and strongly agreed that they adopted Selected Response in assessing their Social Studies students. The results also showed that teachers’ qualification has no significant influence on the modes of assessment of junior secondary school students in Social Studies (χ2 = 335.15, p>0.05). The study concluded that Selected Response (SR) is widely used by teachers as a mode of assessing the junior secondary school students in Social Studies and students’ performance in Social Studies is considerably moderate.