In this work, the thermal radiation effects on magnetohydrodynamics (MHD) natural convection in 2D cavity and 3D cavity are investigated numerically with different Plank number PI and Hartmann number Ha. The radiative flux is obtained by solving radiative transfer equation (RTE) instead of widely used approximation methods. A combined method SCM-ACM (spectral collocation method-artificial compressibility method) is developed to solve the N-S equations. Comparative analysis of thermal radiation effects on MHD flow and heat transfer in 2D cavity and 3D cavity is conducted with same parameters. The results show that the thermal radiation strengthens the flow and heat transfer on the spanwise direction in 3D cavity, while the magnetic field plays opposite effects. The differences on flow and temperature distribution between 2D cavity and 3D cavity (on the middle XY-plane in the spanwise direction) are remarkable for high level thermal radiation and weak magnetic flied, and become smaller for weaker thermal radiation and stronger magnetic flied. (C) 2017 Elsevier Ltd. All rights reserved.