This thesis mainly studied the education on personality in the child education held by Toegye. Through this process, we expect to understand what Toegye especially emphasized in his education on personality. Toegye left numerous letters throughout his life, being 3,154 letters in total. Addressees were mostly literary people, and about 1,300 letters were sent to relatives including his son and grandson. 516 letters were sent to his eldest son Lee Jun (李 寯, 1523∼1583) and 125 to his grandson Lee An-do (李安道, 1541∼1584). The main content of his letters sent to his son and grandson was about daily lessons. In other words, Toegye showed his figure as a considerate but sometimes strict father and grandfather instead of being a Confucian scholar passing on great discussions or theoretical teachings in his letters. Contents on personality in Toegye‘s letters to his son and grandson all show a common inclination. He pointed out to his son and grandson in detail to set an aim (goal) in life, remember one's manners as a classical scholar, thoroughly abide by the law and principles, and to fulfill one's basic duties as a human being. What's interesting is that these lessons all belong to the area of ‘self-discipline(修己)’. He asked to deepen one's character through studies, to behave and compose oneself, abide by the law and principles to uphold justice, and to enhance morality by do what is right and fulfilling duties as a person.