Following Yamada and Karo [J. Phys. Soc. Japan 63 (1994) 289], we have calculated the frequency-field diagram of antiferromagnetic resonance of KCuF3 using an eight-sublattice model and mean field approximation. A spin flop is found to take place from the [110]p to a direction perpendicular to [110]p in the c-plane at H≌ 42172A-m, which is in good agreement with the data obtained by measuring magnetic susceptibility.