Abstract: A theoretical method for computing radiation heat transfer of the in-cylinder soot in diesel engines is developed based on Rayleigh scattering theory. It shows that the in-cylinder radiation heat transfer is proportional to the product of in-cylinder soot mass and the fifth power of the radiation temperature. The model doesnt include any adjustable constants or parameters for specific conditions and it is much terser than the classic models, which greatly simplifies the analysis of the in-cylinder soot radiation heat transfer in industrial furnaces, especially in diesel engines. Because the parameters of the model are local, the model can be used as local model as global model by different algorithms. Multidimensional simulations of a direct-inject diesel engine were made to test the model as a global model. When the same input data was used, good agreement between our model and the classic model. The stability of our model was also validated.