Abstract: The vitrinite reflectance is the most important maturity indicator. Due to the geological factors and measurement technique, the measured vitrinite reflectance is different from that of expected. In upper Paleozoic source rock from Santanghu Basin, both of the vitrinite reflectance enhancement and suppression can be observed. The vitrinite reflectance enhancement is caused by the volcanic activity which was widespread laterally but effected vertically only in a small scope. The vitrinite reflectance suppression resulted from high content of sapropelinite. Using hydrocarbon index, the correction was made and the maximum suppression is about 0.3%.