The characteristics and the performance limit of a steam injector have been investigated. The disruption of a 5 mm diameter water jet that flowed coaxially into a pipe of 7, 10 or 20 mm diameter where vapor flow existed and the heat transfer of vapor condensation on the jet surface were examined. The heat transport to the radial direction in the jet was stronger than that of the turbulent flow in a pipe. The condensation heat transfer was mainly dominated by the heat transport in the water jet since the vapor condensation heat transfer efficiency was quite high. If the Kelvin-Helmholtz instability wave length was used as the jet disruption criterion, it predicted the jet disruption conservatively. It was proved that the steam injector tested could elevate the delivery pressure to five times higher than the suction-side water pressure in the present experimental condition.