A detailed description of flow rate measurement technique for automotive exhaust is presented. The system consists of a sector field mass spectrometer for continuous analysis of helium concentration in the exhaust gas and a mass flow controller which injects pure helium at a constant rate into the intake manifold of an engine. The exhaust flow rate can be calculated by helium injection flow rate dividing by the concentration since the concentration value is a measure of the ratio of helium dilution taking place in the engine. The advantages of the technique consist of (1) no disturbance from strong pulsed flow present when an engine is idling, (2) easy time alignment with gas analyzers, and (3) measurement of dry based flow rate that can be directly multiplied by dry based gas concentration to obtain mass emission rate. Errors in the measured flow rate could arise from several factors such as (1) time of diluted helium to travel from a helium injection point to an exhaust sampling point, (2) existence of helium in the ambient air, (3) mixing, and (4) analyzing capability of the mass spectrometer. Experiments have been conducted to investigate degrees of contribution to the error from above factors. As a result, calibration of the technique requires ambient helium compensation and vacuum pressure compensation. Comparison with CVS-bag method for CO2 mass emission from FTP hot transient mode showed correlation within 1%.