Abstract: The century between Haussmann's Paris reconstruction and the ambitious planning schemes in the 1940s witnesses not only the emergence and development of modern city planning, but also the rise, popularization and materialization of historic town conservation ideas, and the scale and scope of conservation extended from individual buildings to the city as the whole. Mechanical innovation and historic conservation become the intertwined characteristics of the new discipline of modern city planning, highlighting comprehensiveness as the key note in the course of development of the field. Based on the work of the German School, British planners collectively made remarkable contributions, with the emergence of the idea and practice of country conservation and new principles of dealing with the historic portion of the city in the 1920s. In the 1930s, two Charters of Athens established the comprehensive methods of conservation in an urban context, some of which were realized in the post-war reconstruction planning schemes towards the end of WWII. Investigating the history of ideas and practices relating to historic conservation within the fabrics of the development of modern city planning as a new discipline helps us better understand the connection between the two, providing new clues for studying city planning history.