To clarify the way to estimate the performance of the steam injector (SI), experiments were performed. The axial pressure distribution in the steam injector and the delivery pressure were measured. The pressure in the condensing section was negative, decreased as flow proceeded and reached minimum at the throat. Then the pressure recovered to high pressure at the diffuser section. The larger the subcooling of the water flow was, the larger the pressure decrease at the condensing section was. The larger the pressure decrease at the condensing section was, the higher the delivery pressure was. The delivery pressure prediction model that has been proposed by other researchers was properly predicted the dependency of the delivery pressure on the steam flow rate. However, some discrepancy between the prediction and the experimental results was noticed about the dependency of the delivery pressure on the water flow
