This paper presents an analytical investigation on the performance of optimal beamforming scheme in multiple-input multiple-output dual-hop amplify-and-forward systems. We first derive a set of new closed-form expressions for the statistical properties of the resultant channel matrix, including the cumulative distribution function and probability density function of the maximum eigenvalue, as well as their first order asymptotic expansions. These analytical expressions are then applied to derive some important performance metrics of the system, i.e., outage probability, diversity order, array gain and ergodic capacity. Numerical simulations are carried out to validate the correctness of the analytical results.