The educational theory of E. Key, 'pedagogy from child, is founded on the understanding of children's nature. For the deeper understanding of Key's educational theory, therefore it is necessary to explore the theoretical foundation of Key's understanding of children's nature. In this respect, this study aimed to explore the psychological foundation of Key's understading of children's nature. The contents of this study are summarized as follows. First, the main stream of psychology in the end of nienteenth century and the beginning of twentieth century was examined. As influential streams, experimental-developmental psychology and psychoanalytic-depth psychology were introduced. Second, the relationship between Key's understading of Childre's Nature and two psychological streams was explored. Ideas obtained from two psychological streams were presented in her works, especially in . In the book, many psychologists were mentioned or their actual texts were quoted. The author who the greatest impact on Key's psychological aspect was W. Preyer. And then, the characteristics of Key's psychological understanding of childre's nature were discussed. Key's psychological understanding of ㅊhildre's nature was evaluated and then the infuluence of her work was assessed. Finally it was mentioned that Key could not make her own psychology, however she adopted an independent position and assumed her own critical standpoints.