The noise equivalent power of optical heterodyne detection at 10.6 μm has been measured with a method based on Raman-Nath diffraction of a CO2 laser beam. One of the frequency shifted first order diffracted beams is used as the signal radiation. The local oscillator radiation is obtained by splitting off a part of the laser beam incident upon the device used for the acoustooptic diffraction. The signal power can be varied over a large dynamic range by changing the acoustic input power. A study of the probable errors shows that the total error in the NEP measurement is less than 30%.