Abstract: A linear pre-coding algorithm is proposed for the amplify-and-forward MIMO relay system with imperfect channel state information. In the proposed scheme, optimal relay precoding matrix is deduced based on the assumption that the full channel state information (CSI) of the source-relay link is acquired at the relay and the estimation errors and the antenna's correlation information of the relay-destination link is considered. The linear processing matrix at the receiving terminal is first derived based on the minimum mean squared error (MMSE) criterion and then the closed form of the optimal relay precoding matrix is deduced with the maximum power constraint at the relay station. The numerical results show that the proposed scheme achieves better performance in terms of both bit error rate (BER) and mean squared error (MSE) compared with other schemes.