Abstract: With advances of oil and gas exploration,methodology of petroleum resource assessment has been focus on analysis of geological features and on statistics of the exploration data instead of the material ba lance by geochemical data.Application of fractal method to the petroleum assessment may open new idea to geologists,because field sizes of the discovered oil and gas accumulations and the field number are fol lowing a kind of box structure in terms of fractal.This paper introduced a simple fractal method to esti mate undiscovered hydrocarbon reserve on a published data set of the Minnelusa play in the Power River basin,U.S.A.The author believe that the method is applicable in estimation of either petroleum re sources or reserves in China.