The present invention relates to a multiphase flow metering system and apparatus for measuring the relative volumetric proportions and temperature of constituents in a fluid mixture passing through a pipe. The apparatus includes a cylindrical cavity structure having a first microwave sensor system with a first loop antenna is coaxially placed around the pipe and a rectangular cavity structure having a second microwave sensor system with a second loop antenna is vertically placed across the pipe. The first and the second microwave sensor system, both placed external to the pipe, transmits microwave signals at a first and a second predetermined resonant frequencies. A computer system analyzes the shifts in frequencies, based on the permittivity and temperature of the constituents, to identify the relative volumetric proportion of the constituents in real-time.