The \JKM> state selection of a seeded CH3I/Ar supersonic beam has been achieved using a 2-m electrostatic hexapole field. Several \JKM> states were better resolved than the previous one in a CH3Cl/He supersonic beam. A voltage-modified trajectory simulation was carried out for fitting the observed focusing curve, in which the first-order approximation of the Stark effect was corrected in terms of the hexapole voltage in an empirical formula. A better agreement was found on the peak positions of the focusing curve, and the relative heights of the three peaks suggest that CH3I molecules in the present beam populate among the \JKM> states in a statistical way with a rotational temperature of 3 K.