Steam injector (SI) is known as passive jet pump and heat exchanging device which operates without external power source or mechanical machineries. It utilizes direct contact condensation heat transfer between steam and water-jet as a driving mechanism of the operation and is capable of discharging sub cooled water at higher pressure than the inlet fluids pressure. In addition, it has an excellent heat transfer capability, more than 1000 times that of shell and tube heat exchanger. In the present study, thermal hydraulics characteristics of the water-jet-centered supersonic SI system were investigated from both experimental and analytical approaches. The SI body was manufactured with stainless steel equipped with overflow port. The water injection nozzle was designed with shaft-driving mechanism to freely adjust the axial location of the water nozzle and steam inlet cross sectional area. High pressure steam was supplied to the SI from once-through boiler, which is capable of supplying saturated steam at the maximum pressure of 0.63 MPa for the current test facility. Water jet was injected at mass flow rate of 0.4-0.8 kg/s. Pressure and temperature measurements were conducted at inlet and outlet of the steam injector system as well as at the overflow port to investigate the operation characteristics of the SI. Results showed the water-jet centered SI's promising functionality as a passive coolant injection in view of its quick-start up, operable condition limits, discharge pressure and heat transfer capabilities at current inlet conditions. In addition, obtained experimental results were compared with analytical model to assess the predictive capability of discharge pressure value and reasonable agreement was obtained. (C) 2016 Elsevier Ltd. All rights reserved.