It is important to be able to measure accurately theQfactor of an open resonator to compute loss tangent of low‐loss dielectric materials. In this paper, we report the results of the heterodyne detection technique used to measure theQfactor of an open resonator in theW‐band frequency range. A more general algorithm is used for determining theQfactor. This algorithm offers some advantages over the commonly used ‘‘half‐power frequencies’’ method of measurement of theQfactor. The effect of signal/noise ratio on the accuracy of measurement of theQfactor is experimentally studied. It is shown that with the set up used, it is possible to measure theQfactor to an accuracy of better than 1%.