The interaction collision cross-section of the microscopic particles is one of the key parameters for obtaining the internal properties of particles and the transmission performance among them[1].Usually,molecules in the excited vibrational states have a larger collision cross-section than that of the unexcited molecules in the ground state,for which to the natural abundance methanol gas,however,our light-induced drift[2] experimental and calculation results have shown the inverted results.The relative difference in collision frequency and in collision cross-section between particles in the excited state and ground state have been found to be negative by measuring the magnitude of light-induced drift.By considering of intermolecular Stockmayer potential averaged over the orientation of the polar to methanol molecule[3],the relative difference in diffusion coefficient which is proportionally related to the relative difference in collision cross-section is calculated,in which the relative change of the dipole moment of methanol is the main term and is found to be negative.The calculated results are consistent within the range of experimental error with the measured dipole moment values of methanol from Stark splitting observations.