The boron powder with diameter from 0.3 μm to 3.4 μm was made with planet ball-milling. The particle size of boron powder was characterized by SEM and Laser Particle Size Analysis. DTA and TG were used to characterize the thermal behavior of the 50/50-B/AP composition with different boron particle size. The results show that thermal behavior of boron composition is affected obviously by the particle size of boron powder. The smaller the particle size of boron powder,the greater the apparent decomposition heat and mass loss of the 50/50-B/AP composition. Compared with the original boron(d₅₀=3.40 μm),heat release of fine boron(d₅₀=0.30 μm) is increased from -19.7 J·g^-1 to 556.2 J·g^-1.