This study compares the results of the petroleum resources of China evaluated in the U.S. Geological Survey World Petroleum Assessment 2000 (USGS WPA 2000) with those evaluated in the China National Petroleum Assessment 2007 (CNPA 2007). The USGS WPA 2000 reported the mean undiscovered petroleum resources of China to be 12.12 BBO and 85.79 TCFG, which is a much lower estimate than the 107.38 BBO and 692.13 TCFG assessment reported in the CNPA 2007. Six major factors, including petroleum resource classification systems, data sources, assessment scopes, unit divisions, assessment methods, and assessment parameters, contributed to the differences in these two assessments. Reserve growth and undiscovered resources are two independent parts of total petroleum resources according to the definition in the USGS WPA 2000, whereas undiscovered resources of the CNPA 2007 included estimates of reserve growth. The USGS WPA 2000 showed a much higher minimum field size than the CNPA 2007 did, and only six Chinese basins were covered in the former, whereas 115 Chinese basins were evaluated in the latter. For the same basins, unit divisions of the USGS WPA 2000 also differed from those of the CNPA 2007 because of their different data sources and exploration and exploitation experiences. Different methods used by these two agencies also affected their assessment results to some degree.