The purpose of this case study was to explore the effects on the students English writing ability and affective attitudes when applying collaborative learning to English writing classes for college students. Sixty-three university students (30 in an experimental group and 33 in a control group) participated in the case study. During the ten-week period of the case study, the control group students participated in the individual writing activities, while the experimental group students participated in a collaborative learning group of five students. Writing tests were conducted before and after the experiment to examine the effect of collaborative learning activities on writing ability, and independent sample t-tests and paired sample t-tests were performed for statistical verification. In addition, to examine the change of affective attitude, the online survey was conducted only on the experimental group, and a paired sample t-test was performed. Results of the study showed a significant improvement in the English writing ability of the experimental group that applied collaborative learning. There was also a positive change in affective attitude of the experimental group that performed collaborative learning. Based on the results of this case study, educational implications for collaborative learning on English writing for further studies are suggested.