Exaction of development conversion gain was used as an attachment criterion of land use conversion in Taiwan, that also was a planning control tool to address speculation in land use up zoning. Media always raise many doubts on the definition of development conversion gain. This research reviews every kind of definition and analyzes method. From the point of planning control, this paper estabishes an analysis method and definition of development conversion gain. It suggests the definitions of development gain due to land use conversion is land price appreciation due to land use conversion then decrease the transaction cost of land use conversion.