In solid propellants, aluminum plays a key role in energy performance enhancement, but it also induces combustion problems such as low combustion efficiency and agglomeration. Herein, we study the effects of Al on the combustion properties of low-signal composite-modified double-base (CMDB) propellants. Experiments were conducted to determine the heat of explosion, the burning rates, and the combustion wave structures. The flame structure, the burning surface phenomenon and the morphology of the extinguished surface layer were observed. In addition, distribution of Al particles was calculated. It was found that using small Al particles or lowering the Al content will reduce the burning rate. This result was mainly cause by the "thermal sink" effect of Al. The Al agglomeration and ignition behaviors in the dark zone and the flame zone were observed at 1 and 3 MPa. It was found that propellants containing more than 10 wt.% Al or using Al particles smaller than 5 mu m tended to generate large Al agglomerates. The extinguished burning surface presented "metal skeleton" structure under low pressures. The Al particles smaller than the HMX would be locally concentrated, so that it was easier for them to form large agglomerates. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.