Ultrafine hexanitrohexaazaisowurtzitane (CL-20) samples were prepared by a ultrasound- and spray-assisted precipitation method. Raw CL-20 and ultrafine CL-20 samples were characterized by SEM, FT-IR spectroscopy, XRD, and particle size analysis. The impact sensitivity and thermal stability of two CL-20 samples were also tested and compared. The results indicate that by this recrystallization process, the mean particle size of CL-20 is 470 nm, and the particle size distribution was in the range from 400700 nm. The particle morphology is nearly spheric with a smooth surface. Compared with raw CL-20, the impact sensitivity of the ultrafine sample is significantely reduced and the drop height (H50) is increased from 12.8 to 37.9 cm. The critical explosion temperature of ultrafine CL-20 decreased from 235.6 to 229.0?degrees C, which suggests that the thermal stability of ultrafine CL-20 is lower than that of raw CL-20.