Break off performance of a dual-hop cognitive relay selection system is researched based on decodeand-forward（ DF） protocol. The system uses half-duplex communication mode and all nodes are equipped with single antenna. Also

both the maximum interference power from primary receiver and the maximum transmission power constraint secondary system. Moreover

both the exact closed-form expression for break off probability and high transmission power asymptotic closed-form expressions are derived in non-identical Rayleigh fading channel environments. The correctness of theoretical analysis result is verified via Monte-Carlo simulations. Experimental results show that break off probability is reduced and the system performance is improved as numbers of relays increase

but improvement is limited. Moreover

changing relationship between the maximum interference threshold and the maximum transmission power brings bottleneck to improvement of system.