We report the results of antiferromagnetic resonance (AFMR) measurements conducted on KCuF3 at various frequencies from 3.8 to 10.6 GHz at 4.2 K. The resonance linewidth is first found to depend on the frequency, i.e., the lower the frequency the greater the resonance linewidth, no matter whether the AFMR field is forced on the easy axis or uneasy axis. Moreover, a linewidth peak seems to exist for H 11 [100]p at about 4 GHz. Based on the model of Yamada and Kato (1994 J. Phys. Soc. Japan 63 289) and considering the Laudau-Lifshitz damping term, the result of numerical calculation for the resonance linewidth is in good agreement with the data of AFMR experiments.