For amplify-and-forward (AF) relay systems employing coherent modulation, the receiver at the destination requires the full channel state information (CSI) of the sourcerelay-destination (SRD) links and the amplified relay-destination (RD) links to perform maximal ratio combining (MRC). To reduce the burden of channel estimation and CSI forwarding in AF relay systems, we propose an MRC receiver with partial CSI which needs the CSI of the SRD links and the variance of the source-relay (SR) links. We analyze the bit-error-ate (BER) of the proposed scheme based on the moment generating function approach. Compared with the full CSI case, the BER performance degradation due to partial CSI depends on the channel variances of the SR and RD links. When the channel variances of the SR links are much larger than those of the RD links, the performance loss due to partial CSI is negligible.