The crystal structure of an organic ferromagnet, alpha-HQNN, was characterized by one-dimensional chains constructed through intermolecular hydrogen bonds. Furthermore two parallel hydrogen-bonded chains are connected by bifurcated hydrogen bonds. The analysis of heat capacity data indicates the three-dimensional ferromagnetic intermolecular interaction in this crystal. The results confirm the presence of the ferromagnetic intermolecular interaction along hydrogen bond. The observed isotope effect in the alpha-HQNN-d(2) crystal is consistent with the above interpretation.