It is generally believed that Child-Study contributed to the development of Progressive Education Movement and it had little influence on American Herbartianism except that it forced them to shift their emphasis upon subject matters to that upon children. Since they produced their own Project Method in early 1920's, it is partly true that American Herbartians learned much from Child-Study and changed their way. However, in fact, they positively took part in the Illinois Society for Child-Study from 1894 to 1897, and C. C. VanLiew, who was one of the famous advocates of Herbartianism, served three years as a secretary of the society, while they launched National Herbart Society in 1895 and discussed some educational and instructional theories such as five formal steps, concentration,... and culture-epochs. After all, the period when they were engaged in Child-Study perfectly coincided with the period when they improved their theories of instruction, so it is natural to believe that Child-Study had direct influence on the development of American Herbartianism. In this article, I try to revaluate the role of Child-Study for American Herbartianism, examining some articles in Transactions of the Illinois Society for Child-Study and The Public-School Journal. The psychological principle of apperception, which means the reception of a new or partially new idea by the assistance of kindred ideas already in the mind, was turned out to be a pillar supporting all the theories of instruction. So, in order to demonstrate them, they studied the principle of apperception in Child-Study, investigating the mental condition and the experience of children before entering the primary school and researching the steps of their physical and mental development. It is possible to believe that the role of Child-Study for American Herbartianism was directly important in 1890's.