The geothermometer is one of the most important methods for reconstructing the thermal history of sedimentary basins. Many geothermometers have been employed to study the thermal history of carbonate succession, including organic indicators, such as bitumen reflectance(R_b), CAI of conodonts, reflectance of vitrinite-like macerals, free radicals density, laser-Raman carbon geothermometer; and mineral indicators, such as fission track thermochronology, illite crystallinity, acoustic emission of rocks. However, all these geothermometers have their own scopes of application. Based on thermal simulation results of type I and type II kerogen, this paper discusses the application of free radicals concentration as the thermal indicator for reconstructing the thermal history of carbonate succession. A series of free radicals data were tested under thermal simulation at different heating temperatures and times, and quantitative models between free radical concentration(N) of organic matter and time-temperature(TTI) for type I and type II kerogen were obtained. This N-TTI relation was used to model the Ordovician thermal gradients of Well TZ12 in the Tarim Basin. The modeling result is corresponding to the results obtained by apatite fission track data and previously published data. After all, to study the thermal history of the Lower Paleozoic carbonate succession, which has undergone complex structural movement, we had better employ a number of thermal indicators and geothermometers together.