A focusing curve for the CH3I beam, structured by the JKM] single rotational states, was simulated by a new Monte Carlo simulation that includes the second-order Stark effect and was found to be unexpectedly important. A simple and useful relationship was confirmed between the peak positions of the focusing curve, simulated by the pure first-order Stark effect, and those of the curve simulated by the first- and second-order Stark effects. The rotational distribution of the CH3I beam was found to be a bimodal Boltzmann-type distribution expressed mainly with a rotational temperature of 2.4 K and an additional high-temperature distribution favored for the higher rotational levels.