Supersonic steam injector is a passive jet pump without rotating machine and high efficient heat exchanger because of the direct contact condensation between the supersonic steam and the subcooled water jet. It is considered that flow behavior in the steam injector is governed by the direct contact condensation and complicated turbulent flow with large shear stress induced by velocity difference between the steam and water. However, the studies about the turbulent flow under the large shear stress with direct contact condensation are not enough. Especially mechanisms of the momentum and the heat transfer are not clarified in detail. Objective of the present study is to investigate the turbulent behaviors of the water jet and the interface between the supersonic steam and the water jet. Interfacial wave in the mixing nozzle is observed by using a high speed camera. It is suggested that there are some correlations between the interfacial wave propagation velocity and heat transfer of water jet in the mixing nozzle.