Carbonate source rock research is one of the important problem in petroleum organic geochemistry and in petroleum exploration in carbonate rocks. One of the argument is the evaluation standard of source rock, and the focus is minimum value of TOC of source rocks. Aimed at gas fields of Permian and Triassic carbonate of southwest Sichuan basin, combining gas reservoir geochemistry with petroleum geology, utilizing advanced geochemical analysis, correlating gas with source rock, observing the variation of TOC in vertical section, the article has deduced the minimum value of TOC of source rock in theory. And conversely, based on the geological condition and the background of exploration, calculated the hydrocarbon
source quantity, compared with the prospective resources, the author determined 0.3% as the minimum value of TOC of carbonate source rocks in Permian carbonate of southwest Sichuan basin. By the way, the differences of  mechanism, conditions, influence and process of hydrocarbon formation between the carbonate and clastic rocks is discussed.