Different propellant compositions were prepared by incorporating nano-sized cobalt oxide from 0.25% to 1% in HTPB/AP/Al-based composite propellant formulations with 86% solid loading. The effects on viscosity build-up, thermal, mechanical and ballistic properties were studied. The findings revealed that by increasing the percentage of nano-Co3O4 in the composition, the end of mix viscosity, the modulus and the tensile strength increased, whereas the elongation decreased accordingly. The thermal property data envisaged a reduction in the decomposition temperature of ammonium perchlorate (AP) as well as formulations based on AP. The ballistic property data revealed an enhanced burning rate from 6.11mms(-1) (reference composition) to 8.99mms(-1) at 6.86MPa and a marginal increase in pressure exponent from 0.35 (reference composition) to 0.42 with 1% nano-cobalt oxide.