Hydorxyl Terminated Poybutadiene (HTPB) based Rocket Motor Liner is a kind of Polyurethane Elastomer used to insulate the rocket motor case for a sufficient period of time to allow the propellant to burn completely without degrading the motor case. Carbon Black, which provides flame retardancy and reinforcement, was used as an inert filler. HTPB as the polymeric binder and Isophorone Diisocyanate (IPDI) as the curing agent were used to prepare Rocket Motor Liner. Carbon Black having different particle sizes were used as a filler such as N990 (MT type), N660 (GPF type), N330 (HAF type) and Printex U (RCG type. Carbon Black particles was characterized by measuring BET surface area and z-average size. Various additives were added with Carbon Black in Methyl Ethyl Ketone (MEK) solvent such as BYK 9077(wetting agent), BYK 9076(wetting agent), Triton X100 (non-ionic surfactant), SDS (anionic surfactant), TTAB(cationic surfactant) in order to disperse carbon black particles. In the liner formulation, NCO/OH ratio, triol/diol ratio and amount of carbon black in the total formulation were fixed. In the sets of the formulation, each type of Carbon Black was used alone and then two different carbon blacks (N990 and Printex) were added together to generate formulations having binary filler with the ratio of 1:3, 1:1, and 3:1. Mechanical properties of the various formulations were measured using a Universal Testing Machine. In addition, glass transition temperatures (Tg) of the formulations were analyzed using DSC and DMA, and thermal expansion coefficients of the formulations were measured using a dilatometer. Furthermore, Shore A hardness and viscosity of the formulations were also characterized.