At present time, soot deposit role in the process of transient heat transfer in the diesel engine combustion chamber is studied deficiency. In the paper, a new calculation experimental method of investigation and a new mathematical model are suggested. The new method and model allow evaluating the heat insulation effect of soot deposit. The experimental research of local heat transfer was carried out using a new type of sensors for transient heat flux. Calculated local values of soot deposit show close fit (deviation was about 3 - 9%) to the experimental data. It was specified that soot deposit temperature in normal direction from the heat-loaded surface to the limit depth (detail surface) decreases about 7K per micrometer and heat flux density about 0.5%. Phase delay fluctuations of temperature and heat flux density throughout the soot deposit depth was about Δᵩ = 0.50 of crankshaft angle rotation on each micrometer. The necessity of accounting of soot deposit heat insulation is obvious, because the processes of soot generation and its conversion into the soot deposit are the essential parts of heterogeneous combustion in the diesel engines. The same process but with much less intensity, takes place in the engines with the direct injection of gas.