A new selective transmission mode in a three-node relay system is proposed when interference cancellation (IC) is not applicable on receivers. According to the effective Signal-to-Noise-Ratio (SNR) of each link, the source determines whether to transmit new information in the second slot while the relay node is forwarding the information received during the first slot. By the proposed monitoring mechanism, we can improve the ergodic capacity of the whole system compared to conventional relay protocols without too much feed-back overhead. Moreover, we prove that under careful power control of each node, full spatial diversity and capacity gain will be achieved even when there is no interference cancellation processing at the destination node. Thus we can reduce the burden of the receiver significantly. Two methods with perfect IC are compared as a supplement to explore the flexibility when there is no complexity limit at receivers. Both Decode-and-Forward (DF) and Amplify-and-Forward (AF) modes are analyzed respectively.