Polyurethanes were synthesized by the reaction of hydroxy-terminated polybutadiene (HTPB) and diisocyanates such as toluene diisocyanate (TDI), diphenylmethane diisocyanate (MDI), hexamethylene diisocyanate (HMDI), and isophorone diisocyanate (IPDI). Carbon black and antimonytrioxide (Sb2O3) were incorporated into these formulations as fillers. Dioctyladipate (DOA) and trimethyllolpropane (TMP) were also added as a plasticizer and crosslinker, respectively. These polyurethanes were investigated as inhibitors for composite-modified double-base (CMDB) propellants. Due to superior mechanical properties, thermal properties, and low nitroglycerine absorption, HTPB-MDI-TMP-derived filled polyurethane was selected and evaluated as an inhibitor for a CMDB propellant.