Atmospheric-pressure helium plasma jet is getting much attention especially in plasma medical application or dental application. It is thought that active species play important role in the plasma processes and they are generated from the interaction between the plasma and molecules included in the ambient air and the discharge gas. OH radical is one of the most important active species. However its production mechanism is not yet elucidated completely. In this study, OH density distribution in the vicinity of dry and wet surface in the effluent of an atmospheric pressure helium plasma jet was measured by using laser induced fluorescence (LIF). OH density and its distribution varied depending on some parameters; helium flow rate, gap between the surface and the end of capillary tube and humidity of the objective surface. The maximum OH density was approximately 1.0 ppm in the case of the plasma jet extending toward wet surface when high voltage 10 kV, 8 kHz was applied. The result suggests that OH radical is produced by the dissociation of H₂O nearby the surface that evaporates from the surface or adheres on the surface.