Among the emerging technologies in order to meet ever stringent emission and fuel consumption regulations, Exhaust Gas Recirculation (EGR) system is becoming one of the prerequisites particularly for diesel engines. Although an EGR cooler is considered to be an effective measure for further performance enhancement, exhaust gas soot deposition may cause degradation of the cooling. To address this issue, the authors studied the visualization of the soot deposition phenomenon to understand its behavior. Based on thermophoresis theory, which indicates that the effect of thermophoresis depends on the temperature difference between gas and the wall surface exposed to the gas, a new visualization method using a heated glass window was developed. By using glass with the transparent conductive oxide: tin-doped indium oxide , temperature of the heated glass surface is raised. This new method was applied to an EGR cooler, which successfully enabled real-time visualization without soot deposition on the window surface. Consequently it was visually confirmed that the soot adheres to the heat exchanging surface. This observation also confirms that basic mechanism of the deposition is due to the thermophoretic phenomenon. Furthermore, there were notable findings observed in the experiment of running a diesel engine and visualization model. First, position of water condensation occurrence depends on the local thickness distribution within the soot deposit. In addition, size of the soot peeling area can be effectively increased by inducing high gas velocity with Vortex Generator fin (VG fin) when condensed water exists. VG fin has heat transfer promoting effect by generated vortex along the gas flow direction on the heat transfer surface. In this paper, these observations are discussed in detail as well as the development of the visualization method.