Abstract: Calorimetry was put forward for measuring oil/water ratio inside pipelines and relevant theoretical analysis and test research have been carried out. Result showed that calorimetry is of such advantages as simple, reliable and on any movable parts. Its proposing and research presented a great significance in developing three-phase flow meter of oil/gas/water. It pointed out that a key problem for using this method to measure oil/water ratio lay at accurate determining the uneven factor KT of temperature field at the inlet of the electric heater.