The method for oligomerizing olefin according to the present disclosure is a method for oligomerizing olefin using an oligomerization catalyst system and includes deteriorating the activity of the oligomerization catalyst system by injecting a deactivator in a latter part of a multimerization reaction of olefin. The deactivator may include an additive for polymer containing at least one functional group selected from the group consisting of a hydroxyl group, an amine group and an amide group. According to the oligomerizing method, the isomer of 1-hexene and/or 1-octene and alpha-olefins with C10 to C40 may be decreased via the restraint of the additional side reaction of a product. Since the deactivator is an additive for enhancing the physical properties of a polymer, a separating process thereof is not required, thereby improving economic feasibility and productivity.