Over the last few decades, co-operative learning has been an important area of research in many parts of the world. Research in this period has focused on the effectiveness of using co-operative learning in order to increase student achievement in different subjects, enhance their social development and render learning as an enjoyable everyday social activity. This study has contributed to a deeper understanding of student group leaders'' influence on the group members and the group activities. It also explored the leadership development of student group leaders and different leadership styles displayed during this development. The main findings of the study can be summarised as follows: Student group leaders had mainly focused on motivation of the group members and the organisation of the group activities, their leadership styles developed through training, survival, comprehension, and mastery stages. Three leadership styles identified to have been displayed by group leaders are affiliative, instructional and servant. The study has implications for teaching and learning, teacher education and policy.