The aim of this research is to define the school principals' implicit leadership theory and to reveal its relationship with the proactive behavior. The study is conducted in Bagcilar and Basaksehir districts in Istanbul and the target population of the research is 153 school principals working in state elementary schools, middle schools and high schools in these districts. Samples not used in the research and the research is conducted on the population. 137 school principals participated in the study. Implicit Leadership Scale is used to determine the theories of implicit leadership of school principals and Proactive Personality Scale is used to determine their proactive personality traits. The study concluded that school principals perceive individuals as leaders who have high moral values, are skillful, sensitive, strong and powerful and who can influence his/her colleagues. Another conclusion of the study is that a low level of significant relationship was found between the personal morals, sensitivity, power and impressiveness factors of school principals' implicit leadership theories and their proactive personalities; and a medium level of significant relationship was found between the skillfulness factor of implicit leadership theories and their proactive personalities.