The size and aggregation of soot particulates emitted from a diesel engine have been investigated using a transmission electron microscope, and the chemical composition in soot particulates were analyzed by a gas chromatograph in order to clarify the mechanisms of growth and combustion in the exhaust pipe.In the present report, the following results have been obtained: (1) When the gas temperature was lowered below 400°C in the exhaust pipe, the soot particle distribution shifted to a larger size, from 50 Å to 100 Å. This tendency was remarkable in the case of lower gas temperatures, (2) In this case, polynuclear aromatic hydrocarbons in the soot particulates exhibited a tendency to increase in the downstream of the exhaust pipe. (3) When hydrocarbons, for instance C2H2, C3H8, etc., were added to the exhaust gas, the rate of growth of soot particles increased compared with no addition. (4) When the exhaust gas was heated up to above 600°C, it was observed that soot particulates burnt from their surface, and they became smaller or burnt out. In this case, smoke density decreased and soot particle distributions shifted to a smaller size.