The determination of thermal maturity of the Lower Paleozoic sediments has long been a difficult problem for evaluation of the hydrocarbon source rocks due to lack of vitrinites. Based on microscopic observation of graptolite- bearing shale and laboratory simulated maturation of low mature graptolite- bearing shale and coal, optical characteristics and thermal evolution of the graptolites in marine shale was investigated to anlayze the feasbility of graptolite random reflectance as an indicator of the thermal maturity. The non- granular graptolite of lower Paleozoic marine shale occurs in banded- shaped parallel to the bedding or in the fragmented shape, and has similar optical characteristics as that of vitrinite, therefore, it can be used to measure its reflectance. The relationship between non- granular graptolite reflectance (R′gr) and vitrinite reflectance (R′vr) can be presented as following: R′vr = 1.055R′gr－0.053. The distribution characteristic of thermal maturity of the Wufeng- Longmaxi Formation in the eastern Upper Yangtze area has been determined using the non- granular graptolite random reflectance. The results indicate that the non- granular graptolite random reflectance is a practical indicator for the thermal maturity of the lower Paleozoic sediments and provides a reliable method to determine the thermal maturity of the Lower Palaeozoic sediments.