Security is a crucial aspect in nowadays wireless communication systems. The open nature of wireless makes the communications more vulnerable to eavesdropping, which leads to that the physical layer security (information theoretic secrecy) is becoming attractive due to its relying on the characteristics of the transmission medium. In this paper, we study the secrecy on a gaussian relay-eavesdropper channel with a trusted relay, which is assumed to be able to decode and encode wiretap codes. We discuss several cooperative strategies to guarantee the information secrecy in some cases and bound the corresponding secrecy rate. Also, we derive a cut-set-like upper bound on the secrecy capacity for our scenario. The relative proofs are also presented in this paper.