In this paper, we consider the relay station (RS) selection schemes for an orthogonal amplify-and-forward relaying system with half-duplexing in a multi-cell environment. The selection schemes are to to maximize the instantaneous received SINR either when the channel gains for the other cells are fully known or not. We propose a sub-optimal RS selection scheme, which is given by the harmonic mean of the ratios of the SNRs (signal-to-noise ratios) and INRs (interference-tonoise ratios) among the adjacent cells. Furthermore, we analyze its outage performance. Our analysis presents the performance comparison between the optimal and sub-optimal RS selection policies, including the existing selection schemes. The proposed sub-optimal scheme provides a simple yet useful performance metric to improve the outage performance without resorting to the full channel knowledge in the multi-cell environment.