Abstract: In the molecular structure study, infra-red spectrophotometry is well known for its sensibility, rapidness accuracy and intactness of the sample.This new techmique has been widely used recently in petroleum geology.This paper describes a method of discriminating the crude oil from the polluted,which has been worked out on the basis of analytical data showing that different variaties of refined oil with different specific gravity have different styles of spectrum and different (converted)densities at certain specific wave numbers,and through simulation experiment.The applicability of this method is discuesed.