In order to clarify the spin dynamics of the stable radical crystals, we have investigated the magnetic behavior and spin relaxation of low-dimensional antiferromagnetic organic materials, N-[(3-Nitrophenyl)thio]-2,4,6-triphenylphenylaminyl radical crystals, by cw and pulsed ESR. With decreasing temperature from 298 to 20 K, the line width of the cw ESR spectrum increases. Upon further decrease of temperature, significant exchange narrowing occurs below 10 K. The line broadening is more prominent in the direction of g parallel to than that of g perpendicular to. The temperature dependence of the inverse of the transverse relaxation time, T-2, for the g parallel to direction shows a divergence at ca. 20 K as determined by the FID analysis. This temperature is consistent with the magnitude of an antiferromagnetic interaction of J/k = -26 K. These findings indicate that the spin relaxation and spin dynamics in this system are governed by low-dimensional antiferromagnetic interactions among the radical species.