In order to extend available period of thrust generation for the high performance rockets, modified polyurethane-NH_4ClO_4 base propellants are developed. The main compositions are 22-25 parts of polyurethane and 75-78 parts of NH_4ClO_4, including necessary quantity of additives. The compositions are proved to have easier processing and casting characteristics. The grains cured at comparatively low temperature show satisfactory mechanical properties in elongation, strength, coefficient of expansion and adhesive ability, in spite of containing high percentage solid fillers. These propellants stand comparison with polybutadiene base propellants in performance potential and rubbery behaviours even in low temperature range. The linear burning rate and the maximum available specific impulse which are derived from measured pressures and durations of combustion with the sides burning rocket motor loaded with 600gr. grain are not more than 3mm/sec and more than 240kg/kg/sec at 70kg/cm^2 abs. combustion pressure, respectively. The sensitivity of burning rate with combustion pressure is less than the case of the propellants which contain polysulfide or polyester as fuel binder.