Spherulites of 3-nitro-1,2,4-triazol-5-one (NTO) were grown from cooling crystallization with co-solvent water and N-methyl-2-pyrrolidone (NMP). Solubilities of NTO in NMP + water were measured. The spherulites of NTO consisted of fibrous crystals which were arranged minutely in a radial manner from the center. Optimum conditions for spherulitic crystallization of NTO were set for the cooling rate and the specific compositions of ternary mixture. Spherical spherulites were obtained in the ratio of NTO/NMP ranging from 0.2 to 0.6 and in the ratio of water/NMP ranging from 1.0 to 4.8 at the cooling rate of 10 K/min.