Abstract: Early Palaeozoic marine hydrocarbon source rocks in China are characterized by high maturity, low amount of organic matter, especially lack of structured macerals, which leads to difficulties in determining conventional matruity parameters such as vitrinite reflectance and exinite fluorescence. Objective determination of the maturation level of these source rocks is an urgent matter for oil and gas exploration. In this paper, the authors, having investigated the detailed organic petrological characterstics of Early Palaeozoic marine source rocks, sampled mainly from Tarim Basin and found that bitumen, micrinite, marine vitrinite and graptolite are not only very common in the source rocks, but their reflectance has a good relationship with the depth or maturity. The results of thermal - simulating tests show that bitumen and graptolite reflectance within the "oil window", marine vitrinite throughout the hydrocarbon - generating stages and micrinite renectance after the oil - generating peak are of a near straight linear relation with vitrinite reflectance, and that all of them can be used as reliable maturation parameters.On the basis of this work, a preliminary plan is put forward which is based on the four parameters used to evaluate the maturity of Early Palaeozoic marine hydrocarbon source rocks.The analysis of a series of typical source rocks from Tarim Basin shows that this method not only solves successfully the determination of the maturity of source rocks, but also provides additional important information on oil and gas generation.