This paper presents a novel method for the real-time apparent density measurement of the working fluid in outlet pipes of a stream-injection boiler. The measurement system consists of a vertical measuring pipe, a container with constant height and a differential pressure (DP) transmitter. The working fluid in the upper part of the vertical measuring section flows into the container which is connected to the positive pressure side of the DP transmitter. The negative pressure side is connected to the lower part of the vertical measuring section through a pressleading tube. The apparent pressure measurement is then converted into DP measurement. When the feed water flow is known, the apparent density of the working fluid at the outlet is determined by the measured differential pressure. Results from oil field trials confirm that the proposed method is effective.