The applicability of radiation transfer theory for calculations of the thermal radiation emitted by spherical particle of a semitransparent material, and in particular the determination of radial heat generation profiles, is analyzed. For homogeneous isothermal particles, a comparison with the ex:act solution based on the Mie theory shows that the radiation transfer calculations are sufficiently accurate for diffraction parameter of the particle of x greater than or equal to 20. Numerical examples for large particles illustrate the transition from conditions of dominant radiation of the central region of the particle to conditions of the surface layer emission. A new differential approximation for radiation transfer in a refracting particle is proposed. This approximation called MDP0 (modified DP0) is much simpler than the radiation transfer equation. Using MDP0, we have a chance to consider radiation-conduction interaction in nonisothermal particles without great computational efforts. (C) 2000 Elsevier Science Ltd. All rights reserved.