NTO (3-nitro-1,2,4-triazole-5-one) crystals were obtained from NTO/H2O solution by a cooling method with ultrasound irradiation. The morphology and the size distribution of NTO crystals are found to be a strong function of ultrasound frequency and sonication time. The shape of NTO crystals with a 47 kHz sonicator is cubic or tetragonal. On the other hand, the shape of NTO crystals with a 28 kHz sonicator appears to be orthorhombic. The metastable zone width of the NTO/H2O system is observed to decrease under ultrasound irradiation. Ultrasound effect on the metastable zone width becomes dominant dramatically when the cooling rate is smaller than 0.5 degreesC/min. In particular, at cooling rates smaller than 0.1 degreesC/min, the limit of the metastable zone widths with ultrasound is about 2 degreesC, independent of the saturation temperature.