To explore the detailed information regarding how cooperative learning (CL) functions in a specific situation and to gain knowledge on how to maximize the effects of CL in an English as a foreign language setting, a case study was conducted. The setting of the case study was General English 3 for the sophomores majoring in applied foreign languages at a private university in Taiwan. The data collection of the study consisted of two parts. The first part was a survey using a questionnaire assessing the students’ perceived improvement on eight learning domains and the students’ perceived quality of cooperation; the second part was a qualitative one using in-depth interviews as the tool. The results of the survey showed there were significant positive correlations between the perceived quality of group cooperation and the perceived improvements in four of the eight domains of ability listed in the questionnaire. However, no significant correlation was found between the perceived quality of group cooperation and the perceived improvements in speaking, writing, grammar, or critical thinking. The follow up in-depth interview provided further information regarding the quantitative findings indicating that good group interaction as an immediate cause for improvement on active learning and information searching strategies and good group interaction as a remote cause of improvement on reading and listening. The study also showed that context variables such as the familiarity, the mutual trusts among the students and the social interactive skills of the students all have great impacts on the effects of cooperative learning. Stages of Cooperative Learning for EFL teaching/learning were proposed based on the findings.