Accurate monitoring of a multiphase fluid flow in a dynamic pipeline is a significant problem in the oil industry. For efficient management of oil field wells, a real-time online system with capabilities to monitor fractions of oil, gas and water in oil production pipelines is required. These parameters determine the oil quality and inform how much water, oil and gas is produced from oil wells. This paper reports on the development of a novel non-intrusive sensor, which is based on electromagnetic waves cavity resonator. It determines and monitors the percentage volumes of each of the two phases (oil and gas) in the pipeline using the resonant frequencies shifts that occur within the resonator. A laboratory prototype version of the sensor system was constructed and tested. Experimental results were in good correlation with theoretical model that was simulated with High Frequency Structure Simulation (HFSS) software. Reported system will form the basis for the advanced real-time multiphase fluid composition monitoring platform.