Changes in the mechanical properties of hydroxyl-terminated polybutadiene/ammonium perchlorate-based composite solid propellants were studied during the curing period with respect to variations in the crosslink density, which was predominantly determined by the equivalent ratio of diisocyanate to total hydroxyl (NCO/OH ratio) and the equivalent ratio of triol to diol (triol/diol ratio). For this purpose, 16 propellants were prepared in different compositions through changes in the NCO/OH ratios (0.81, 0.82, 0.83, and 0.85) for each triol/diol ratio (0.07, 0.09, 0.11, and 0.13) and were tested for their mechanical properties immediately after curing. The propellants with an NCO/OH ratio of 0.82 had minimum stress, modulus, and hardness with maximum strain capability, whereas the propellants with an NCO/OH ratio of 0.85 showed just the opposite behavior. Variations in the isocyanate level seemed to have more effect on the mechanical properties at higher triol/diol ratios. It was also concluded that the propellants with triol/diol-NCO/OH combinations of 0.11-0.83, 0.11-0.85, 0.13-0.81, 0.13-0.83, and 0.13-0.85 were not acceptable for upper stage case-bonded rocket applications because of either high tensile strength or high modulus.