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The objective of this paper is to maximize the secrecy rate in an untrusted relay network with imperfect channel state information (CSI). Considering bounded channel estimation error in cooperative jamming network, we optimize the power allocation for confidential and jamming signals to maximize the instantaneous secrecy rate. The upper and lower bounds of achievable secrecy rate are analyzed. The ergodic secrecy rate (ESR) is further derived to evaluate the maximum average secrecy rate. And the optimal power allocation is also studied based on ESR. Numerical results show optimal power allocation can increase both instantaneous secrecy rate and ESR, and at higher SNR, the impact of channel estimation error can be neglected for the maximum ESR, which is asymptotically approaches to the upper bound.