To reveal the influence mechanism of the particle size and crystalline form on the combustion of boron particles, the ignition and combustion characteristics of boron particles were studied by using a laser ignition system. Results show that in the particle size range of 25-65 μm, the ignition delay time of crystalline boron is between 13 and 19 ms, and shows no evident regularity as a change of particle size. The combustion efficiency of both amorphous and crystalline boron increases with increasing the particle size. The oxide layer covered on the surface of boron particles is a crucial factor of the reaction efficiency when particle size is less than 65 μm. The ignition delay time is the shortest and the combustion becomes most intense for crystalline boron with a particle size of 55 μm, which indicates that 55 μm may be the optimum size for ignition and combustion. Amorphous boron has a shorter ignition delay time and higher combustion efficiency, compared to crystalline boron. The burning flame and emission spectrum of amorphous boron are also stronger. This implies crystalline form has a more important influence on boron ignition and combustion than particle size.