In order to explore the relationship between the particle size of nitroamine explosives and their cook-off sensitivity, RDX, HMX and PETN samples with different particle sizes (centimillimeter,micron,submicron) were prepared by spray fine and dripping methods respectively. Scanning electron microscope (SEM) and laser particle analyzer were used to characterize samples. The cook-off thermal sensitivity for RDX, HMX and PETN samples were tested and analyzed by GJB772A-1997 method 608.1. Results show that thermal sensitivities of RDX, HMX and PETN increase with the reduction of particle size.