Steam submerged jet condensation is a direct contact condensation widely used in many industries. However, this process may cause damage to related equipment due to pressure oscillation, especially at low steam mass flux. This study synchronously investigated the steam-water interface fluctuation of unstable condensation jet and its corresponding pressure oscillation through a horizontal pipe at low steam mass flux to research the condensation oscillation mechanism. Three typical flow patterns, namely, Chugging, hemispherical bubble oscillation (HBO), and encapsulating bubble oscillation (EBO), were distinguished based on the dynamic behavior of steam-water interface and oscillation pressure characteristics. Only the dynamic pressure in EBO region was periodic among the three flow patterns. In the EBO region, the pressure oscillation frequency increased with the increase of steam mass flux and decreased with the increase of water temperature. A flow pattern regime map for horizontal jet condensation related to steam mass flux and water temperature was presented based on experiments. Moreover, the influence of pipe diameters on low steam mass flux was analyzed. For the flow pattern regime map at low steam mass flux, the critical steam mass fluxes showed an inverse relationship with pipe diameter. An empirical criterion was introduced to determine the flow regime map at low steam mass flux. (C) 2019 Elsevier Ltd. All rights reserved.