The 10.6μm CO2 homodyne receiving has been adopted for detecting deterioration of the spatial ’coherence of laser radiation. The relation between the heterodyne efficiency and normalization aperture of the receiver is presented. It is shown that the efficiency of the heterodyne receiver decreases with increasing of the normalization aperture when the aperture of the heterodyne receiver is larger than the coherence diameter, but the result is opposite when the aperture is less than the coherence diameter.