The first release of the Sixth National Population Census results in April 2011 revealed that mainland China had a total population of 1339.72 million, of which 665.58 million resided in urbanized areas. This suggested that the urbanization process had accelerated during the period 2000–2010, with an increase of the urbanization rate from 36.09% to 49.68%. However, due to the incomparability of the Censuses data, caution must be taken when urbanists investigate or model Chinese cities by such datasets. This research note summarizes and compares the shifting definitions of city (?,shi) and urban population (????,chengshi renkou) statistics with an emphasis on the Sixth Census, and analyzes their implications for measuring and understanding the urbanization process in China.