By analyzing different understandings of petroleum resources，we discuss the differences in connotations of the various concepts of petroleum resources and put forward two different resource assessment approaches.One is hydrocarbongenerationbased approach which takes hydrocarbon generation units as basic appraisal units.Based on principles of hydrocarbon generation，migration and accumulation，it establishes geological models and mathematical models to calculate hydrocarbon generation amount and eventually total resources.The other is hydrocarbonaccumulationbased approach which takes hydrocarbon accumulation units as basic appraisal units.On the basis of comprehensive analysis of petroleum geology，it figures out the number and size distribution of oil/gas accumulations or cells and then calculates undiscovered resources.We also discuss main factors that influence resource assessment.At last，suggestions are made to improve petroleum resource assessment in China：1）to strengthen research on resource assessment methods and complete petroleum resource assessment system；2）to complete classification criteria and system for oil/gas resource in order to bring them in line with international standards；3）to set up a dynamic resource assessment mechanism and to establish the national team，system and database for petroleum assessment.