An online multi-phase flow meter for continuously and accurately measuring the volumetric quantities of gas, oil, and water flowing concurrently in a conventional feed pipeline, such as an oil well pipeline or a transport pipeline. The online multi-phase flow meter includes: an inlet pipe; an exit pipe; a cylindrical gas separation unit that is connected to the inlet pipe; a gas measuring system; an oil and water measuring system; a slurry collection system; a maintenance system; an electronic controller and solenoid valve for maintaining a minimum liquid level in the cylindrical gas separation unit; and a data acquisition system comprising a microprocessor. Also, a method for continuously and accurately measuring the volumetric quantities of a multi-phase mixture containing gas, oil, and water with the online multi-phase flow meter.