Abstract: Thermal expansion coefficient and dynamic mechanical properties in terms of the storage modulus E′, loss modulus E" and the loss tangent of a crosslinked multiblock copolymer of hydroxy-terminated polybutadiene and polymethyl methacrylate were investigated. E′ and E" were calculated according to the single relaxation time model.The thermodynamic parameters were presented. The molecular weight between crosslinking points M_C was calculated about 945. The characteristic damping behavior shows that this polymer is a semicompatible multiblock copolymer with excellent mechanical damping.