a flame retardant liquid rubber composition comprises a hydrogenated polyhydroxybutadiene polymer and castor oil , which are modified with an isocyanate having the formula ## str1 ## wherein r 1 , r 2 and r 3 respectively represent a lower alkyl group or hydrogen atom and n is an integer of 1 to 4 , and alumina hydrate and magnesium hydroxide . the castor oil can be incorporated before adding said isocyanate or can be converted into an isocyanate prepolymer by reacting with said isocyanate before the addition .