Abstract: The separation and analysis of the diastereoisomers of pristine (2,6,10,14-tetramethylpentadecane) in these highly and overly mature coal samples from the Junggar Basin were carried out by gas chromatography mass spectrometry (GC/MS), and the relativity of the pristane isomerization ratio (PIR) and the reflectances (R_o) of these coal samples were studied. The results show that the PIR keeps quite linear relation with the R_o, and the linearity is up to 0.995 1. Furthermore, verified test with source rocks of the Sichuan Basin was conducted in laboratory scale. It has been proved by experiments that the model has a precision of 0.897 9 in forecasting the test results. The results suggest that the PIR may be used as an new indicator of maturity for the highly and overly mature source rocks, and it provided a new way to identify the maturity of these highly and overly mature source rocks.