During the rapid development of synthetic aperture radar (SAR), jamming techniques against SAR have aroused intensive study interests in the field of electronic counter measures (ECM). In this paper, we compare the jammer's transmitter power required by coherent deceptive jamming against SAR and by incoherent noise jamming against SAR. It is pointed out that, in order to yield same jamming-to-signal ratio (JSR) in SAR image domain, the ratio of the transmitter power of incoherent noise jamming to that of coherent deceptive jamming should be the average pulse compression gain (APCG), rather than, the pulse compression gain (PCG) in SAR image formation processing, which is different from the opinion of some literatures. Theoretical analyses are verified by simulation results.