The pressure broadening coefficient for O 2 in the a 1 Δ g state has been measured to be 2.006+/-0.064 MHz/Torr at 290 K with a submillimeter-wave spectrometer with a backward wave oscillator as radiation source, where the uncertainty indicates one standard error from a least-squares fit. The broadening parameter by nitrogen is also measured. The temperature dependence is examined in the range 246-290 K. These results are useful for deriving the altitude distribution of O 2 a 1 Δ g generated by a photochemical reaction from ozone in the upper stratosphere and mesosphere.