In the present study, we propose a relatively novel wall-fluid molecule collision rule for description of adiabatic solid-wall in atomistic simulations and using it as boundary condition for simulation of rarefied gas natural convection in a finite enclosure of adiabatic side-walls. This novel wall-fluid collision rule keeps the particle total velocity invariant and its normal velocity component reversed with the same magnitude before and after collision, while the other two components can vary randomly. This boundary treatment is more physically reasonable for description of a real solid-wall at adiabatic condition. To examine the performance of this novel rule, natural convection of rarefied gas in a micro-scale rectangular enclosure of length-to-height aspect ratios 2.016 and 4 heated from below is employed as the test model and predicted by direct simulation Monte Carlo (DSMC). The parameters of Knudsen number Kn = 0.01, 0.016, 0.02 and Rayleigh number Ra up to 3061 are considered. The present results demonstrate that the novel collision rule generates physically reasonable predictions of thermal-fluid behaviors at microscales and, compared to the existing boundary treatments of the same class, the present one is more efficient in the computational aspect. (C) 2007 Elsevier Ltd. All rights reserved.