Some of the 4-arylmethyleneamino-TEMPO (2,2,6,6-tetramethyl-piperidin-1-oxyl) radical crystals showed an intermolecular ferromagnetic interaction. The X-ray structure analyses were performed on 4-arylmethyleneamino-TEMPO crystals (aryl = phenyl, p-chlorophenyl, p-biphenyl, p-phenoxyphenyl, p-bromophenyl, 3,5-dichlorophenyl, 3-pyridyl, naphthyl and p-fluorophenyl) at room temperature to investigate whether any systematic intermolecular interactions were demonstrated among these crystals. No special interactions except van der Waals interaction were observed in these crystals. As for the arrangements of N-O radicals in the crystals, two-dimensional O ... O networks are constructed, of which forms are various. A sheet-like arrangement of the N-O radicals is considered to be essentially important for the magnetic interactions, although the O ... O distances are long for the direct interaction between radicals.