Submillimeter wave AFMR measurements of Mott insulator kappa-(BEDT-TTF
)(2)Cu[N(CN)(2)]Cl have been performed at 1.8K in the frequency region
from 50 to 383GHz using pulsed magnetic field up to 16T. The results
resemble the calculated results of the molecular field theory by Pincu
s with easy-plane type anisotropy including the Dzyaloshinsky-Moriya i
nteraction. Observed AFMR modes are analyzed by this model.