Kinetic studios were carried out on urethane polymerization reaction of hydroxyl terminated polybutadiene (HTPB) with hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI) and tolylene diisocyanate (TDI) These results have shown the relative reaction rates of hydroxyl terminated polybutadiene and diisocyananates are TDI>HDI>IPDI. The mechanism of the reaction of isocyanates with compound containing an active hydrogen can be explained by the positive charge on carbon atom of isocyanate group. The reactions were a good second order reaction process with nearly constant activation energy of 8.3-11.5 kcal/mole by Arrhenius equation.