Pyrolysis of BC dry chemical fire extinguishing powders which are useful for Class B and Class C fires was conducted on a thermogravimetric analyzer with sample loading of 10-25mg under dynamic air atmosphere. The effect of particle sizes (medium value 48.99, 27.24, 4.93 mu m) and heating rates (10, 15, and 20 degrees C min(-1)) were examined. The pyrolysis kinetics of the samples was analyzed using a distribution activation energy model. It was found that the decomposition temperature decreased and the pyrolysis rate increased after the samples were milled. The agglomeration of particles during production did not have an appreciable influence on the pyrolysis process of the samples in our experimental conditions. The activation energy value was 77.13-219.78kJmol(-1), 58.18-288.67kJmol(-1), and 44.59-209.17kJmol(-1) for the powder of particle size 48.99, 27.24, 4.93 mu m. We should use micro powder in fire extinguishing.