In Cognitive Radio (CR), primary users have exclusive privilege to access their licensed channels. Therefore, a transmission failure occurs when one or more primary users access the licensed channel during a transmission by users in secondary radio network (SRN). We define such channel access by primary users as return of primary users. Transmission failure caused by return of primary users may not be neglectable if we consider the bursty nature of source data in large amount of networks, especially for multi-hop communication due to the large transmission duration. Furthermore, existing wireless communication systems are facing challenges on flexibility to bursty dynamic data traffics. Aiming to develop solutions for above tasks, we propose a cognitive frequency sharing (CFS) approach for transmitting packets of primary radio networks (PRNs). Under cooperation of SRN, plural independent PRNs share their idle spectrum resources (usually referred as to spectrum holes) in an opportunistic manner. CFS causes no influence to transmission independence within each PRN. Simulation results verify the effectiveness of CFS in improving both the throughput of PRNs and condition of spectrum holes that are for SRN's use.