Abstract: The optimal precoder vector at a source node is designed for MIMO cooperative relay networks with an amplify-and-forward (AF) strategy. Firstly, this paper derive the optimum solution of the precoder vector with the criterion of maximizing the sum of SNR observed at both destination and relay nodes in the first time slot. Meanwhile, the precoder optimization with the criterion of maximizing SNR after MRC at the destination node is also devised, but its analytical solution is unavailable. In order to reduce the complexity, two suboptimal methods are proposed, namely, the gradient descent iterative method and the approximate solution method. The simulation results show that the proposed optimal precoders using two criterions can also improve the system performance, and the case of maximizing SNR at the destination node is more effective because it can describe the status of the receiver directly. The performance of the gradient descent method is almost the same as the one of the approximate solution method, but the former method is more complex due to iterations. The approximate solution method achieves low complexity with no performance reduction.