The influence of adsorbed CO diffusion on the first order phase transition point (y2)of CO oxidation based on Langmuir-Hinshelwood mechanism was studied in detail by Monte Carlo simulation. It was found that CO diffusion increased the value of the first order phase transition point(y2) and when the ratio(k) of diffusion probability of adsorbed CO to adsorption probability of CO is 100, y2 is equal to 0.642. The correlated y2 value is in good agreement with the theoretical result(0.65) obtained by Mai when k approaches to infinity large.