An experimental study on the interfacial behavior of stable steam jet condensation in a rectangular mix chamber was conducted. By high-speed imaging and image processing procedure, the interfacial wave was observed and discussed. From correlation analysis, interfacial wave velocity was obtained, and an empirical correlation for predicting the interfacial wave velocity was also established. Besides, interfacial fluctuation characteristics and corresponding mechanism were also analyzed. The results indicated that, interfacial wave observed in present work was proved to be a manifestation of K-H instability, and the interfacial wave velocities, which were in the range of 25.1-130.4 m/s, were proportional to steam plume penetration length, while inversely proportional to axial position. High frequency component of the interfacial height fluctuation was induced by the interfacial wave, while low frequency component was caused by waving of the whole interface. The results in present work would be significant to understanding the interfacial behavior of steam jet condensation and would be useful for a better design and safe operation on the TFSI. (C) 2017 Published by Elsevier Ltd.