During the process of recrystallizing ultrafine HMX by gas antisolvent (GAS) method,the ultravioletvisible detector was used to measure the supersaturation degree of HMX in CO₂ and acetone system at 10-35 MPa,and the nucleation rate equation was used to calculate the nucleation rate of HMX. The ultrafine HMX was characterized by scanning electronic microscope (SEM). Results show that the supersaturation degree of HMX in CO₂ and acetone system increases with increasing of the pressures. Also,the nucleation rates of HMX are very high,can reach to 10²⁴ orders of magnitude. From results of SEM,there are some obvious differences in particle morphology,size and size distribution of HMX. Analyzing from the view of nucleation rates,there is more energy consumed in nucleation stage at relatively high pressure (35 MPa) than that at low pressure (5-15 MPa),which contributes to formation of HMX particles with regular morphology,small size,narrow size distribution.