Abstract: Under different test conditions, the particulate matter deposited on the inner surface of the heat exchange tube of the EGR(Exhaust Gas Recirculation)cooler will form a deposition layer with different surface microstructures. In this paper, two parameters, the characteristic area ratio of the surface structure of the deposition layer and the fractal box dimension number, are used to analyze the effect of hydrocarbons concentration on the deposit surface microstructure. It is found that the precipitation of volatile substances such as hydrocarbons in the exhaust gas would affect the size of particulate in the gas and change the number and size distribution of the deposit surface structure such as concave and convex. Moreover, with the increase of hydrocarbons concentration, the area ratio of convex structure on the deposit surface increases gradually while the area ratio of concave structure changes insignificantly. The fractal box dimension number of the deposit surface increases gradually with the increase of hydrocarbons concentration in the test gas.