In the current curriculum reform of Hong Kong, the Curriculum Development Council (2002) recommends that teachers make use of co-operative learning to cater for their students' individual differences. A common feature of co-operative learning is that students help each other to learn in small groups towards a common goal (Cohen, 1994; Johnson & Johnson, 1999). For such kind of co-operative learning, the success of members of a group is interdependent to one another (Kagan, 1994; Slavin, 1995). The concept of co-operative learning is radically different from competitive learning in which a student's success builds on the failure of others. This paper aims to investigate the factors influencing teachers when they consider using co-operative learning in their classrooms. A questionnaire was administered to a sample of teachers (n = 179) from five primary schools conveniently chosen. Descriptive statistical analysis as well as factor analysis was performed on the data collected. The results suggested that the respondents agreed, to different extent, to all the twenty items of constraints as listed in the questionnaire. Four categories of factors that influenced teachers' decision to use co-operative learning were extracted. They represented the school and system levels, teachers' conceptions of teaching, pupils' ability and discipline, and classroom level. The study concluded that, in view of the various constraints, it would be difficult for teachers to implement co-operative learning to meet the curriculum re-structuring. Based on the findings, some recommendations were made.