The magnetic behavior of stable free radical crystals ofN-(arylthio)-2,4,6-triarylphenylaminyl are described as studied by the magnetic susceptibility and ESR measurements. The temperature dependence of the magnetic susceptibility and the ESR spectrum were examined from 1.7 K to room temperature. When the temperature is decreased from 298 to 2 K, the line width of the ESR spectrum increases enormously below 5 K in the case ofN-[(2,4-dichlorophenyl)thio]-2,4,6-tris(3-chlorophenyl)phenylaminyl radical crystals, 1. The susceptibility measurements of 1 show the intermolecular ferromagnetic (FM) interaction among the radical spins. The productXmolTincreases with lowering of the temperature. The Weiss constant, Θ, obtained for the Curie-Weiss law, is +5.7 K, indicating ferromagnetic intermolecular interaction among the spins. The magnitude of the FM interaction is fairly large, compared with those of other purely organic ferromagnets.