Abstract: In the World Petroleum Assessment 2000 project,U.S.Geologic Survey(USGS) adopted the method of probabilistic multiplying,namely Monte Carlo simulation to obtain the forecast for undisco-vered petroleum resources.This method is applied to the resources assessment of the Marib-Shabwa Basin in Yemen,and the results show the undiscovered resources will be 102 million tons of oil equivalents(TOE).In the study area,the syn-rift play is considered to be mature,while the fractured basement play is supposed to be highly promising.