A single-line hydroxyl tagging velocimetric system for gas flow is constructed. Hydroxyl tagging line is generated through the photodissociation of H2O molecules in flow by a 193 nm pulsed ArF excimer laser beam. A laser sheet from a pulsed frequency-doubled dye laser operating at 282 nm is used to reveal the OH tagging line through fluorescence. Two time consecutive images are acquired to calculate the velocity distribution of flow field. The lifetime of OH photoproducts photo-dissociated by 193 nm laser in air and flame are studied. The velocity measurement for room-temperature air flow and high-temperature supersonic flow are carried out, and the basic analysis and discussion of experimental results are performed.