Abstract: Polyclic aromatic hydrocarbons(PAH)in source rock from Wells A 3 and Sai 1 of the Erlian basin ofNeimongol autonomous Region are taken for examples.In this article alkylnaphth-alene and alkylphenathrene homologs of the short chains, flouranthene, pyrene, mephylpyene, benz(a)anthracene; perylene, etc.are identified and quantified by GC and GC-MS. Pimanthrene(1，7-dimethylphe-manthrene)，retene(1 -methyl一7-isopropyl phenanthrene)and 1-methyl-7-isop-ropyl-1，2，3，4-tetrahy drophenanthrene regarded as representative materials of high plant lipide have been found in different depths of PAH in source rock.The relative contents of alkylphenanthrenes(phenanthrene, C₁一Phenanthrene,C₂一phenanthrene, retene, etc.)exhibiting gradual changes with increasing depth in aromatic fraction of sedimentary rocks from the two wells in different depths are calculated and revealed in this article.It seems that the changes can reflect the maturity of source rocks.The unclear changes of relative contents of fluran thene, pyrene, etc.R-ith different depths are discussed as well。