Abstract: Based on the forming mechanism of diadamantane,the thermal evolution relation among isomers as well as the actual GC-MS detection results of diadamantane-type compounds in the saturated hydrocarbon components of T-J source rock from the Shuanghu area of Tibet,five diadamantane indexes are put forward.By correlation analysis with R^O,it is suggested that methyldiadamantane index(MDII) can be applied as a new maturity parameter for source rock.On the basis of thermal modeling experiments with the lignite (R^O=0.46%) from Baise,Guangxi,the thermal evolution rules of MDII are discussed.MDII has good positive correlation with R^O,and their correlation coefficient is 0.83.As the index for the thermal maturity of organic matter,MDII is applied within the limits of mature and high mature stages.