In America, curriculum became a public issue from the late part of nineteenth century. According to the curriculum thought which significant scholars proposed to meet the social needs in this period, four curriculum traditions are divided, which are: humanists, developmentalists, social efficiency educators, and social meliorists. Humanists emphasized the function of mental discipline subject matter made. Developmentalists maintained education should adapt to children's natural order of development. Social efficiency educators stressed the performance of adult life effectively. Social meliorists thought that the schools were good means to improve the society. These four curriculum traditions became the theoretical constituents of curriculum reform. The American curriculum history in twentieth century was the dialectical process of development that this four curriculum traditions involved. This paper aims to explicate the significant curricular events and thoughts of four curriculum traditions, and to depict the dialectical process of development formed by the American curriculum reform.