Abstract: An easy and economic method to separate alkylated napthalenes and alkylated phenanthrenes from aromatic hydrocarbons was introduced in this paper.The GC-MS tests showed that this method could provide a complete and enriched separation of alkylated napthalenes and alkylated phenanthrenes from crude oils and sedimentary samples,solving the problem that several methyls were difficult to determine accurately by GC-MS tests because of heavy co-elution and laying the foundation for free carbon isotope tests.As an application,this method can be used for aromatic hydrocarbons of monomer hydrocarbon carbon isotope analysis of pre treatment.