The optimal operation pressure within Crookes radiometer is studied using experimental and direct simulation Monte Carlo (DSMC) method. The rotational rates of the rotor in the Crookes radiometer versus pressure are measured using a freely rotating rotor with two different dimensions, one is called a 20 mm rotor, which the side length of rotor blade is 20 rum, and the other is called a 10 mm one. The experiment results indicate that the optimal operation pressure varies with the side length of rotor blade and appears at 0.1 Pa and 3 Pa in the cases of 20 mm and 10 mm rotors, respectively. The pressure difference distributions of gas on the back and front surfaces of the rotor blades are simulated by DSMC method. It is indicated that the maximum pressure differences appear at 0.1 - 0.5 Pa in the case of the 20 mm rotor, and at 2 - 5 Pa in the case of the 10 mm rotor, respectively. The results are consistent with the experiment results. The velocity vector fields of gas within Crookes radiometer with the 20 mm rotor under four different pressures are also presented. (c) 2005 American Vacuum Society.