The present invention relates to a commercial reaction process for the selective oligomerization reaction of ethylene, and a method for efficiently removing reaction heat to be generated in a reaction and regulating the temperature of the reactor and, more specifically, to an oligomerization reaction process of ethylene by circulating a liquid mixture in the reactor, separating unreacted ethylene from this circulation flow by an apparatus for separating unreacted ethylene from the circulation flow, and then cooling the remaining mixture and reintroducing it to the reactor, thereby removing the reaction heat in the reactor and allowing temperature control.