Abstract: In this paper, the mechanism of radiation heat exchange for high temperature pebble beds is studied by view factor, and the analytical solution for view factor between two unit spheres is derived. Compared with various numerical methods for view factors, Monte Carlo Method（MCM） is selected with high accuracy and good parallel performance. As blocked by other particles, the view factor between particles approximately is reduced to zero at distance of double sphere diameter. With spatial local partition by Voronoi tessellation, for body center cube（BCC）, face center cube（FCC） and random packing all particles interacted by thermal radiation are included among double Voronoi spheres, about 60~80 spheres. Within acceptable range many particles with extremely low view factor are neglected, numerical radiation model is greatly simplified.