Carbonization mechanism of bamboo was investigated by means of Electron Spin Resonance (ESR). The results show: (1) Around 600 *C is a key carbonization temperature for bamboo. Below 600C, the first stage, bamboo mainly experiences the thermal decomposition. Above 600C, the second one, the conduction of bamboo charcoal is changed into semiconductor from insulator. Obviously silicon effect is observed in the transition stage. (2)Thermal decomposition of bamboo is mainly concerned with radicals from the decomposition of hemicellulose, cellulose, and lignin in bamboo. (3)Moisture and the carbonized atmosphere have effects on the properties and relaxation mechanism of spin centers.