Mid-range vinylidene content polyisobutylenes were prepared by using a Friedel-Craft type catalyst. The polyisobutylenes with 40 to 70% of vinylidene content were obtained when a complex catalyst composed of n-propanol as a cocatalyst and BF3 as a main catalyst was used at a reaction temperature of −33 to 33° C under a pressure of 3 to 50 kg/cm2. The vinylidene contents were easily varied by adjusting the reaction temperature.