X-Band and millimeter wave esr studies of (Di-DCNQI)2Ag

X-band and millimeter wave ESR measurements have been performed on (DI-DCNQI)2Ag single crystals in the temperature region from 1.8 to 300 K. The static magnetic field up to 1T is used for 9.4 GHz and the pulsed magnetic field up to 16 T is used for the millimeter wave region from 40 to 315 GHz. Electron paramagnetic resonance was observed above 5.5 K but the antiferromagnetic resonance was observed below 5.5 K. This result is consistent with the previous suggestion by 1H-NMR measurements that the system undergoes antiferromagnetic order below 5.5 K. The spin-flop field and the magnetic anisotropy of the system are discussed from the frequency dependence of the antiferromagnetic resonance at 1.8 K.