PURPOSE: To obtain the title composition excellent in blooming resistance, low- temperature flexibility, weather resistance, etc., by adding a specified amount of a (sulfur) compound and a specified amount of a vulcanization accelerator to a random copolymer rubber having a specified composition and specified properties.

CONSTITUTION: The composition is prepared by adding 0.5-10 pts.wt. sulfur or sulfur compound and 1-20 pts.wt. vulcanization accelerator to an ethylene/ propylene/7-methyl-1,6-octadiene random copolymer rubber which has an ethylene/propylene molar ratio of 60/40 to 85/15, an iodine value of 7-32, an intrinsic viscosity of 1-4dl/g as measured in decalin at 135°C, a Tαβ/Tαα intensity ratio in an NMR spectrum of: 0.5 or below, a B value of 1-1.5 wherein the B value is calculated from the NMR spectrum data according to the equation wherein PE is the molar fraction of the ethylene contained in the rubber; PO is the molar fraction of the propylene contained in the rubber; and POE is the rate of number of the propylene/ethylene chains to the total number of the diad chains and a glass transition temperature of -53°C or below.