Monte Carlo simulation was carried out for the adsorption of CO an beta -Mo2C with various C/Mo atomic ratios. The CO adsorption on beta -Mo2C showed the minimum value of the self-diffusion coefficient and the maximum value of the adsorption energy at C/Mo = 0.40, in accord with the experimental data of total CO uptake an the beta -Mo2C0.82 (C/Mo = 0.41). The molecular dynamics of the CO adsorption predicts the approach of CO to the surface of beta -Mo2C. The CO molecule preferably adsorbs on carbon deficient sites in the top layer (the molybdenum atom is in the second layer) and molybdenum atoms neighboring the carbon deficient sites of beta -Mo2C. The simulated molecular dynamics suggested that the CO molecule hopped from the molybdenum atom to the carbon deficient site in the top layer, being displaced 90 degrees by each turn.