Method for oligomerising an a-olefin

A method for oligomerizing an alpha -olefin in the presence of a catalyst comprising a specific chromium compound including chromium atom and pyrrolyl group and an alkylaluminum compound, wherein the alpha -olefin is contacted to the chromium compound and the alkylaluminum compound without preliminarily contacting the chromium compound to either the alpha -olefin or the alkylaluminum compound.