It is necessary to comprehensively consider the natural property and economy of the mineral resources before the determination of mineral resources boundary,in order to make reasonable use of economically valuable mineral resources and at the same time ensure the economy of mining.Based on the big resource notion,post mining operation cost is introduced to replace overall cost.Mineral resources boundary is determined through 3 steps,that are to calculate the cost after mining operation,to calculate the cut-off grade of mineral resources and to determine the average grade that is close to the cut-off grade of resources using geological model of resources.The minimum of the average grade that is close to the cut-off grade of resources is the one to determine.The method is applied in a open-pit gold mine,and accurately delineates the maximum boundary of mineral resources,can report the mineral reserves in the project,is beneficial for mineral resources geologists to determine reasonable cut-off grade of mineral resources,delineate and estimate mineral reserves in a proper way.