Thailand is entering into a new phase of educational administration under the new National Education Act of 1999 which requires all educational institutions to have internal and external quality assurance in order to ensure improvement of educational quality and standards at all levels (Section 47). This means that all educational institutions, especially in the private sector, have to undergo many changes, including changes in administration, teaching and learning processes, methods of evaluation and professional development programs. The current study focused specifically on the transformational leadership behaviour of principals on school culture. Culture is considered to be a key factor in determining whether school improvement is possible (Deal & Peterson, 1999). The study was conducted in two phases: quantitative and a qualitative, and carried out in the secondary schools of the Brothers of St. Gabriel, in Thailand. In the quantitative phase, the researcher used two instruments, namely the Multifactor Leadership Questionnaire (MLQ, Short form) developed by Bass (2000) to measure the leadership behaviour of principals, and the School Culture Survey developed by Gruenert (1998) to measure school culture. The results from the quantitative study were used to select a school for the second phase of the study that was carried out using a qualitative approach. The quantitative findings provided descriptive evidence that transformational leadership existed in the schools of the Brothers of St. Gabriel, in Thailand and the dominant culture of the Gabrieltie schools was Professional Development. In addition, the findings also revealed that there was a significant relationship between leadership behaviour and school culture. In the second phase, the leadership behaviour of the selected principal was interpreted based on transformational leadership applied in a school setting. This process was outlined by Leithwood (1996) as frame of reference to identify a pattern of leadership behaviour of the selected principal. The findings revealed that the selected principal exercised the qualities of transformational leadership to some extent. Some qualities were missing and some were practised to a moderate degree. The study helps provide guidance and direction to principals who wish to exercise their leadership in a more appropriate and relevant way particularly in a context of change.