Abstract: Unconventional resources gradually become the important domain of oil and gas exploration in China, in which the evaluation of source rock, especially the total organic carbon content (T_OC), is critical due to the “source and reservoir in one body”. Conventional evaluation methods of T_OC based on logging data are all indirect ones, and calculated results involve some uncertainties as they are affected by various influential factors. In this paper, inelastic gamma spectra and capture gamma spectra were recorded with the gamma ray detector, and elemental contents in formation were derived with spectral analysis technique. After that, the total inorganic carbon content (T_IC), which is calculated with the T_Ic -related elemental contents, was deducted from total carbon content (T_C) to directly determine T_OC. According to the characteristics of shale in Longmaxi and Qiongzhusi formation of Sichuan basin, Monte Carlo method was employed to build 6 formation models, gamma spectra under different formation conditions were simulated, and then the simulated gamma spectra were analyzed with elemental standard gamma spectra. The simulation results demonstrate that the method mentioned in this paper can directly provide high-accuracy results of T_OC, and the relative error of calculated value is less than 6% when T_OC is more than 2%.