In this paper the OH radicals produced by a needle-plate negative DC discharge in water vapor,N2 + H2O mixture gas and He + H2O mixture gas are investigated by a laser-induced fluorescence (LIF) system.With a ballast resistor in the circuit,the discharge current is limited and the discharges remain in glow.The OH rotation temperature is obtained from fluorescence rotational branch fitting,and is about 350 K in pure water vapor.The effects of the discharge current and gas pressure on the production and quenching processes of OH radicals are investigated.The results show that in water vapor and He + H2O mixture gas the fluorescence intensity of OH stays nearly constant with increasing discharge current,and in N2 + H2O mixture gas the fluorescence intensity of OH increases with increasing discharge current.In water vapor and N2 + H2O mixture gas the fluorescence intensity of OH decreases with increasing gas pressure in the studied pressure range,and in He + H2O mixture gas the fluorescence intensity of OH shows a maximum value within the studied gas pressure range.The physicochemical reactions between electrons,radicals,ground and metastable molecules are discussed.The results in this work contribute to the optimization of plasma reactivity and the establishment of a molecule reaction dynamics model.