Disclosed herein is a method of preparing an EP(D)M elastomer, which includes pre-cooling a reactive solvent to −80 to −100° C.; and polymerizing ethylene, at least one higher α-olefin having 3 to 18 carbons, and selectively, at least one conjugated or non-conjugated diene having 5 to 15 carbons, in the presence of the reactive solvent. According to the method of the current invention, the yield of the EP(D)M elastomer is drastically increased even though a conventional preparation device is used unchanged. Further, methods of controlling the yield of the EP(D)M elastomer and of recovering the EP(D)M elastomer are provided.