This study investigates the effects of corpus-based cooperative learning on learners’ understanding and expression of English grammar rules and the affective domain. For this purpose, a total of 43 middle school students in the 3rd grade participated in the study and were classified into three groups: deductive grammar learning, corpus-based cooperative learning, and corpus-based individual learning. The results of the study revealed that all the groups presented a statistically significant improvement in learners’ learning grammar. However, there was no significant difference in learning grammar rules between groups. In respect of understanding and expression of English grammar rules, the groups of corpus-based cooperative learning and corpus-based individual learning showed statistically significant improvement. In the affective domain, the corpus-based cooperative learning group had a positive effect on positive attitude area, especially on using corpus in learning grammar. The results of the study suggest that the modification of corpus data should be considered for low-proficiency English learners and cooperative learning could be a good source of using corpus in the classroom to lessen the psychological burdens of the learners.