Theoretical investigation on the behavior of boron particles in the flow field of a ducted rocket secondary combustor is presented. The study was motivated by the difficulties in achieving good combustion efficiencies of boron particles. The equation describing the gas flow field and the particle behavior were solved numerically. It is presumed that the ignition and combustion of boron particles can be controlled by King's model. The solution presents the trajectory and oxide layer thickness of the boron particles due to the interactions with the surrounding gas. Experimental investigations were done on a connected-pipe test bed. Decreasing the side-inlet angle increases combustion efficiency.