This study was carried out to investigate the effects of cooperative learning strategies on the vocabulary skills of 4th grade students. The study was also designed to ascertain the attitudes of the students in the experimental group towards cooperative learning. Out of 96 4th grade students enrolled in the private school where the study took place, 48 participated in this study. In order to find out whether there was a significant difference between the vocabulary scores for the experimental group and the control group, a pre-test and a post-test were administered to both groups. The data gathered from the students' lesson diaries was used to find out what they thought about cooperative learning strategies. In addition, the teacher of the experimental group was interviewed to verify the results obtained from the lesson diaries. The data gathered from the pre and post tests, lesson diaries and the teacher interview was analyzed both quantitatively and qualitatively. The data obtained from the pre and post tests was analyzed with the Mann-Whitney U test and the Wilcoxon Signed-Ranks Test, while the data gathered from the lesson diaries and the teacher interview was subjected to content analysis. The findings revealed that there was a significant difference between the results of the experimental group and the control group on the post-tests. The experimental group had a higher score on the post-test than the control group. The analysis of the lesson diaries also indicated that the students' positive attitudes towards cooperative learning strategies increased progressively during the study. Lastly, the analysis of the teacher interview confirmed that the students' positive attitudes towards cooperative learning strategies during the study were genuine.