The present invention relates to a method for oligomerization of ethylene, comprising the steps: a) feeding ethylene, solvent and a catalyst composition comprising catalyst and cocatalyst into a reactor, b) oligomerizing ethylene in the reactor, c) discharging a reactor effluent comprising linear alpha-olefins including I-butene, solvent, unconsumed ethylene dissolved in the reactor effluent, and catalyst composition from the reactor, d) separating ethylene and 1-butene collectively from the remaining reactor effluent, and e) recycling at least a part of the ethylene and the 1-butene separated in step d) into the reactor.