Theoretical studies of the ferromagnetic property of phenyl-CH=N-TEMPO were carried out. The effective exchange integrals (J(ab)) were calculated for the nearest neighbour molecules in the crystal and the ferromagnetic interaction was revealed. To study the role of the hydrogen atoms, modified pair models were considered. It was concluded that the bridged beta-hydrogen atoms dominantly contribute to the ferromagnetic interaction in the phenyl-CH=N-TEMPO crystal.