The influence of the large-scale vortexes formed in the mixing layer of two concentric liquid jets upon initial deformation of gas-liquid interface of the outer jet was experimentally examined. The velocity ratio of the concentric liquid jets was changed, and the wavelength and amplitude of the liquid jet surface for each velocity ratio were measured. As the velocity ratio increases, the surface deformation of the liquid jet appears more upstream and its amplitude increases. However. The wavelength has a little dependency on the flow velocity ratio.