Pressure 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 pressure oscillation in steam jet condensation in water flow in a vertical pipe. The interfacial characteristics of the jet plume are acquired by high speed camera, and the pressure oscillation due to condensing jet are captured by using high frequency pressure transducers. Four main types, including Chugging, Oscil-I, Oscil-II and Stable condensation regimes, are identified visually based on the interfacial behavior of the jet plume, and their distribution is described in a three-dimensional condensation regime diagram based on steam mass flux, water temperature, and Reynolds number of water flow. In the Chugging regime, the high-amplitude pressure oscillation appears at low frequency, and the unimodal PDF demonstrates that the pressure oscillation is only dominated by steam mass flux. In the Oscil-I regime, the pressure oscillation is approximately sinusoidal and its intensity is the highest among the four condensation regimes, whereas the intensity of the pressure oscillation remains at a low level and varies little in the Stable regime. In both the Oscil-I and Stable regimes, with increase of water temperature and Reynolds number of water, the unimodal PDF spreads out over a wider range, and finally the bimodal and symmetrical PDF appears for the Oscil-I regime. The statistical analysis shows that both the deviation and maximum of pressure signals could identify the four condensation regimes well, while both the skewness and kurtosis of pressure signals could easily distinguish the Chugging and Oscil-II regimes. (C) 2017 Elsevier Ltd. All rights reserved.