For over a decade, the school education system of Malaysia has been undergoing reforms, including teacher professional development, the quality of teaching and student learning outcomes. The ultimate purpose of these reforms is to establish Professional Learning Communities (PLC) at schools. This goal, according to international researchers, can be achieved by teachers collaborating with co-workers on school improvement projects. However, the educational authorities have encountered many difficulties related to the implementation of PLC, such as the inability to distribute policy information to school leaders due to ineffective communication channels, cultural barriers, underestimation of the value of collaboration by teachers and principals, lack of data for the assessment of the factors affecting collaboration in different settings and contexts among others. Besides, there is a dearth of academic research on the kinds of collaboration that exist in private and government schools in Malaysia. This qualitative, multi-case study aimed to explore the factors that enable and impede collaboration among secondary teachers at two private schools in Sabah, Malaysia. Three data collection instruments, face-to-face semi-structured interview, observation of collaborative meetings and document analysis, were employed to collect the data to answer the main research question: What factors influence secondary teachers of different subjects to collaborate on school improvement projects in two private secondary schools in Sabah, Malaysia? Analysis of the data revealed the participants’ perceptions of collaboration and the factors that influence collaboration at the school. According to the participants, the most influential factors are sufficient time for collaboration, full support of school leadership, flexibility of collaboration, relevance to teachers’ learning, professional communication and consideration of the Malaysian languages. The findings also reflect the challenges of initiating and sustaining meaningful ...