Thermal accommodation of inert He gas atoms colliding on a nanometer-sized Ar or N 2 particle was analyzed using molecular dynamics simulations. The instantaneous values of apparent thermal accommodation coefficient (α th ) showed a distribution close to the normal distribution but with a longer tail toward lower values. The mean and standard deviation of α th for Ar particle were about 0.39 and 0.54, respectively, and 0.44 and 0.56 for a N 2 particle. Those values were almost independent of gas temperature or pressure, with less than 10% variation over a three- or four-fold variation of the gas conditions. The thermal accommodation coefficient per collision (α 0 ), which was calculated from the apparent α th and the average number of collisions on the particle surface, was about 0.18 on the Ar particle and 0.20 on the N 2 particle, both of which are in good agreement with theoretical predictions based on single interactions between free molecules.