To further understand the effect of tobacco stems in tobacco blend, a detailed comparative study of chemical components of tobacco leaves and stems before and after combustion was carried out. Pyrolysis of tobacco leaves and stems was carried out by a CDS Pyrolyser 2000 coupled to a Finnigan 8000(TOP) Gas Chromatograph and a Voyager Mass Spectrometer (GC-MS). A modified apparatus of pyrolysis was used to emulate the behavior of combustion of cigarette. A set of pyrolysis conditions was also developed that approximated those occurring in the pyrolysis region of the burning cigarette. The conditions included heating the sample at 20 degrees C/ms from 30 degrees C to 300 degrees C, 600 degrees C and 900 degrees C in air, and their pyrolysates were analyzed by GC-MS. The results indicated that the values of routine chemical constituents in tobacco leaves and stems were obviously different. The pyrolysates belonged to numerous classes of compounds such as hydrocarbons, alcohols, phenols, aldehydes, ketones, nitriles, alkaloids etc. The kinds of pyrolysates of tobacco leaves and stems were positively related to the pyrolysis temperature levels. On the other hand, at the same temperature level, the types of the pyrolysates from the tobacco leaves were much more than those from the tobacco stems.