Interfacial oscillation associated with direct contact condensation of steam jet in water pipe flow is of high significance for industrial processes. In this paper, experimental study is conducted to reveal the mechanisms of the interfacial oscillation in steam jet condensation in subcooled water flow in a vertical pipe. The interfacial behavior of the jet plume is acquired by high speed camera, and the entire interface in both space and time simultaneously is quantitatively analyzed with digital image processing technology. Bubbling regime occurs at subsonic conditions, where an undulated bubble plume forms with one or more large bubbles falling off intermittently. Jetting regime happens at transonic or supersonic conditions, where a quasi-stable jet plume is observed with numerous uncondensed tiny bubbles continuously shedding off from the end of the jet plume. Distinct waveforms of the radial interface position in both space and time simultaneously are recognized for the two typical condensation regimes. The interfacial unsteadiness reaches a maximum near the sonic point and then drops in the supersonic region, which exhibits similar trend with pressure oscillation induced by condensing jet in a qualitative sense. The results confirm that the pressure oscillation is highly relevant to the motion of the jet interface and it should be induced by the interfacial oscillation. The spatial growth rate of the interface unsteadiness along the jet decreases steadily as steam mass flux increases from subsonic to sonic point and it almost does not vary in the supersonic region. The Kelvin-Helmholtz instability mechanism is more important near the nozzle exit, while in other region the Kelvin-Helmholtz instability is of the same importance as the Rayleigh-Taylor instability. (C) 2019 Elsevier Ltd. All rights reserved.