This study aims to examine the effect of cooperative learning instructional methods (STAD) and conventional learning instructional methods on achievement and attitudes in the subject of Mathematics in Primary Schools. The objective of this study is to detect an instructional method of learning that can improve the achievement and attitudes of students since primary school in the subject of Mathematics. This study applies a time series quasi-experimental design. The research instruments used were pre-test, test 1, test 2, post-test, attitude questionnaires 1,2, and 3, and interview. The data analysis approach was evaluated using a one-way ANOVA to determine the differences between the experimental and control groups. The study findings for pre-test achievement for the cooperative group (n=40) were M = 38.20 (SD = 8.321), test 1 was M = 44.75 (SD = 8.20), test 2 was M = 55.28 (SD = 7.03), and post-test was M = 66.53 (SD = 9.48) while the mean conventional group (n=40) for pre-test was M = 42.20 (SD = 10.80) 1 min test M = 41.75 (SD = 10.07), 2 min test M = 41.78 (SD = 8.78) and post-test mean M = 43.33 (SD = 9.74). The findings of the questionnaire were questionnaire 1 M = 43.10 (SD = 9.17), questionnaire 2 M = 56.05 (SD = 3.47), and questionnaire 3 was M = 63.70 (SD = 3.79) for the treatment group while the control group was the questionnaire 1 M = 40.33 (SD = 10.63), questionnaire 2 M = 45.44 (SD = 8.63) and questionnaire 3 mean value M = 48.71 (SD = 7.66). Findings of the study through interviews also showed that there were significant differences in favor of the cooperative group in student achievement and student attitudes. The results showed that students in cooperative learning classes outperformed conventional learning students.