We have studied the magnetic properties of a series of 2,2,6,6-tetramethylpiperidin-1-yloxyl (TEMPO) radicals, among which eight ferromagnets and six metamagnets were found. Based on their crystal structures, a mechanism of intermolecular ferromagnetic coupling was proposed. The ferromagnetic interactions in the crystals of 4-(p-methylthiobenzylideneamino)-TEMPO and 4-benzylamino-TEMPO were explained by the proposed mechanism. The ferromagnetic transition of 4-(p-chlorobenzylideneamino)-TEMPO was confirmed by zero-field mu SR measurements.