A cw-OPO laser system has been used to measure pressure broadening coefficients for the v(4) vibration-rotation transitions of CH3F by using Stark modulated Lamb dips. Eight pressure coefficients were determined from various pressure data, where each line shape was fitted to the Lorentzian function. The (r)Q(1,0) coefficient was found to be largely different from that of R-r(0,0), where the upper states of both transitions have the same J and K values but different parities of the l-type doublet. The coefficient for (r)Q(7,4) obtained in the present study agrees with that obtained by a Voigt profile analysis (Cartlidge and Butcher, 1990). (C) 2014 Elsevier B.V. All rights reserved.