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An experimental investigation on effects of aluminum particle size on the combustion characteristics of aluminized composite propellant was performed. Reducing the agglomeration size on the burning surface is important to reduce slag in the solid rocket motor and increase the combustion efficiency. Four propellants (HTPB 15 wt%, Al 17 wt%, and AP 68 wt%; AP distribution is trimodal) with the same composition except for different in aluminum particle sizes were examined. Average particle sizes of 30 μm, 10 μm, 5 μm, and 2 μm were considered. Combustion tests were examined with an N2-flashed strand burner with two windows. A high-speed imaging technique was used to observe the formation and ejection of agglomerations on the propellant burning surface at the atmospheric pressure. The agglomeration sizes and its distributions were measured. All distributions were monomodal, and the mean and peak diameters of the agglomerations reduced with decreasing diameter of ingredient aluminum particles. The propellant strands were burned at pressures ranging from atmospheric pressure to 5 MPa. The burning rate of propellants were measured on the basis of the movement of the combustion surface acquired by a video camera. The propellant with 2 μm aluminum particles showed a smaller agglomeration size and higher burning rate (17% higher at 5 MPa) than did the other propellants. 2 μm aluminum particles seem to be effective for decreasing agglomeration size.