© 2019 by authors, all rights reserved.The aim of this study is to determine the fundamental principles required for school principalship that has been defined as a profession. Sequential transformative research design was adapted as the research approach of this study. Sequential transformative research design is a kind of mixed method in which the qualitative data is prior to quantitative data. Firstly, focus group interview with 33 participants were conducted with the participation of a school principal and ten teachers from three different schools. Data were analyzed through content analysis technique. Secondly, Professional Principles for School Principalship Scale (PPSPS) was developed and administered to the members of seven different non-governmental professional organizations founded by educators. The survey was administered to 328 participants. As a result of this study, 30 professional principles were identified in two groups expected to attribute to the school culture. Ten principles in first group are general assumptions (beliefs) and twenty principles are value-based personality standards (values) and the principles of the practice. School principals are expected to base their thoughts, views, attitudes, decisions and actions and take them as reference while performing the school principalship.