Bimodal packing arrangements can be used to overcome the balling phenomena in selective laser sintering (SLS). However, little attention has been paid to the effects of bimodal packing structures on radiative transfer in the SLS powder beds. In this study, a sequential addition packing algorithm is firstly employed to generate 3-D random packing of opaque, diffusively or specularly reflecting spherical particles with the same or different sizes. Then, a Monte Carlo based ray tracing algorithm is formulated, for the first time, to simulate the radiative transfer in the bimodal random packing structures composed of particles with different sizes and emissivities. The credibility of the computer code is verified with published experimental data. A comparison is also made between the calculating results and those obtained by two-flux model. By using the present algorithm, the radiative heat fluxes at both levels of particle and entire bed as well as the transmitted laser energy are statistically evaluated. The influences of bimodal size distribution and particle surface emissivity on the radiative transfer process are examined. Such information is expected to be helpful for optimizing the SLS process. (C) 2009 Elsevier Ltd. All rights reserved.