Disclosed is a cognitive radio system, resource allocation apparatus thereof and method therefor. A resource allocation apparatus comprises: a transmission opportunity assessment device which is used to assess availabilities of transmission opportunities in radio resources of a primary communication system; a vector production device which is used to set a transmission opportunity selection vector for each secondary user in the cognitive radio system based on assessment results of the availabilities of transmission opportunities, wherein the transmission opportunity selection vector includes information for identifying multiple transmission opportunities assessed available; and a sending device which is used to distribute the transmission opportunity selection vector to the secondary user.