Abstract: Thermal decomposing curves of gallic acid in different heating speeds were obtained in the nitrogen atmosphere by TG-DTG techniques. Two analysis methods of Achar and Coats-Redfern were used to speculate the probable mechanism of thermal decomposing reaction and the kinetic parameters. The shelf life of gallic acid at room temperature was calculated by the kinetic parameters of the first stage. With the increasing of the heating rate, thermal decomposing tempreture of gallic acid rose. The most probable kinetic mechanisms of the three-stage thermal decomposition were all chemical reactions, and the corresponding mechanisms followed on reaction order. In accordance with the data of TG and Gaussian, one molecule of gallic acid lost 0.5 atom O at the fisrt-stage, and lost another 0.5 atom O and a molecule CO2 at the second stage. The shelf life of gallic acid at room temperature was about 1.5-2 years.