Abstract: Monoaromatic sterane could be separated as biomarkers with basic peaks of m/c 239, m/e 253 and m/e 267 from aromatic hydrocarbon in bitumen extracts of source rocks or crude oil.Using GC/MS technology,monoaromatic sterane could be identified and its distribution characteristics could be obtained.By monoaromatic sterane we may correlate oil-oil or oil-source rocks,and evaluate their maturity.It is necessary to regard monoaromatic sterane as an additional parameter io steranes and terpancs. particularly for those samples with higher maturity and lower strength of sterenc and terpane. In the present report, purer monoarometic storanes have boon separated by col-amn chromategnaphy.Absorbers are 3 H₂O- Al₂O₃ ( acid ) and fine H₂O content ) , their weight ratio 8:1. This report presents some mass spectrograms and mass chromatograms, and dis-casses the characteristics of monoaromatic sterane mass spectrogram and the strn-cture, distribution characteristics and the source of monoaromatic steranes.Three scries of monoaromatic sterane with basic peaks of m/e 239, m/e 253 and m/e 267 respectively, have been identified, which are from nonmarine crude oils of one oilfield at home.The relative strength of all monoaromatic sterane fragment ions and molecular ions are quite low except the basic peaks.It means that monoaromatic sterane hydrocarbon framework is stable, and carbon-carbon bond crackings are mainly in side chains.For this crude oil, C ring is just the aromatic ring of mono-aromatic steranc.It is proved by mass spectrogram that three identified fragment ions of alkylbenzene, namely m/e 91, m/e 105 and m/e 119, are obviously present. It shows the existence of an aromatic ring.The position of the aromatic ring depends upon whether there are the fragment ions of m/e 145, m/e 159 and m/e 173 or not.As these fragment ions are not present on mass spectrogram, there is no tetra- line ring structure. Then we can exclude the possibility of B or A ring as an aromatic ring.Anyhow C ring is just the aromatic ring.Each of the three series of mon-oaromatic sterane of this crude oil has one or two or three methyl on A or B or C ring.The structure of monoaromatic sterane could be drawn from the above description (refer to figures ) .By mass chromatogram, these three series of monoaromatic sternne are divided into two groups of compounds with low or high carbon number respectively due to the fact that they have undergone different heat evolution periods. Taking account of the monoaromatic steranc with sterane and terpane parameters, this report correlates some oil sources and evaluate the maturity of one section.The mass chromatogram shows that there is relationship between Jing No.16 crude oil and An No.29 source rock. Similar is not only their distribution characteristics.but also all of their parameters.It proves that crude oil of ES₃ of this well is from ES₃ source rock.We have made monoaromatic sterane identification for four nonmarine source rocks in different depths of lower Cretaceous in a basin in the north of China, and discussed the heat evolution regularity, too.It is found that the relative content of monoaromatic sterane with low carbon number increases gradually along with depth, while that with high carbon number decreases.It shows that the low carbon number monoaromatic steranc is stable and high carbon number monoatic sterane urstab1e. And the latter is converted into low carbon numberarom with the depth. So the relative ratio of low carbon number could become the .