Pressure dependence of vinylperoxyl radical formation in the reaction of the vinyl radical with molecular oxygen has been Studied using a laser photolysis/cavity ring-down spectroscopy technique. The absorption spectra of vinyl radical and the product in the reaction were measured in the same spectral range from 439 to 465 nm. Varying the third body carrier gas of Ar from 10 to 120 Torr had obvious effect on the production of the vinylperoxyl radical.