Abstract: Maturity is the geochemical parameter that shows the relationship between crude oil and evolution stage of organic matter of source rock,which is the base of oil-source correlation,so evaluating method is very important.Effective appraisal range and applying condition need to be considered to use maturity parameters.Through overall evaluation on maturity by parameter of light hydrocarbon,aromatic hydrocarbon and sterane terpane,according to the relative high or low of maturity,the Tertiary oils in the Bozhong Sag of the Bohai Bay Basin are divided into two kinds:high-mature condensate oil and mature oil.High-mature condensate oil has high paraffin index,heptane value and methyl phenanthrene ratio,which mainly distribute in the southwest of the sag.Mature oil has contrary parameter characteristic.The result also shows there are multi-fountain source and multi-period reservoir in the Bozhong Sag.