An antitheft method and system for motorcycles are provided. The antitheft method and system for motorcycles authenticate an electronic identifier when power-up of a motorcycle or engine start-up is sensed and cut off the power-up or engine start-up if the power-up or engine start-up is not authenticated. An electronic control unit can generate a theft alarm signal in case of unauthenticated power-up or engine start-up. The motorcycle engine is automatically stopped if the engine is started through an illegal method, and thus the motorcycle is prevented from being stolen. Further, a user can confirm the electronic identifier and start the motorcycle without inserting a key into a key cylinder, and thus the user can start the motorcycle easily and rapidly.