Abstract: Cotton stalk and wood scraps were burnt in a tube furnace to generate soot under different combustion conditions. Soot particles were sampled and detected by TEM, EDS and GC-MS to study their physicochemical properties, then the formation mechanics of soot during biomass burning was deduced. The results show that the typical morphological structures of soot are capsule-like, spherical, catenulated and reticular. Combustion conditions enfluence the burning process and result in the different morphology of the soot. The soot particles collided and coagulated during nucleation and growth of soot, which leads to formation of complicated clustered particles. During biomass burning the soot is mainly generated from pyrolysis of cellulose, which contains furfurans, phenols, aldehydes, furans, alkanes and alkenes. The formation mechanics of soot has been speculated. During burning of biomass, the chemical bonds of cellulose fractured and restructured, which generate CO, CO₂, residual carbon molecule fragments, and so on. Then residual carbon goes on a series of reactions such as reforming, dehydration, carbonization and bond-breaking to generate aldehydes and ketones. And these compounds polymerized and cyclized to form benzene ring structure, and further converted to toluenes and phenols.