Departing from the narrowband channel model, more and more researchers have recently considered the application of multiple-input multiple-output(MIMO) systems over frequency-selective fading channels in combination with the orthogonal frequency-division multiplexing(OFDM) technique.In this paper, we focus on the capacity of MIMO-OFDM systems and how to increase the capacity.It has been proved that the capacity of a MIMO system with space-dimensional water-filling power allocation is greater than that of the system with uniform power allocation.In MIMO-OFDM systems, there is an additional degree of freedom to allocate power with respect to MIMO systems:the OFDM sub-channels in frequency-domain.With the singular value decomposition(SVD) applied to the channel frequency response matrix, the two-dimensional water-filling power allocation algorithm for MIMO-OFDM systems is proposed based on the Lagrange method, and the corresponding capacity analysis formula is derived.Simulation results show that the capacity of a MIMO-OFDM system with the proposed two-dimensional water-filling power allocation is greater than that of the system with uniform power allocation and also greater than that of the system with the one-dimensional water-filling power allocation. Departing from the narrowband channel model, more and more researchers have recently considered the application of multiple-input multiple-output (MIMO) systems over frequency-selective fading channels in combination with the orthogonal frequency-division multiplexing (OFDM) technique. , we focus on the capacity of MIMO-OFDM systems and how to increase the capacity. It has been proved that the capacity of a MIMO system with space-dimensional water-filling power allocation is greater than that of the system with uniform power allocation. In MIMO-OFDM systems, there is an additional degree of freedom to allocate power with respect to MIMO systems: the OFDM sub-channels in frequency-domain. With the singular value decomposition (SVD) applied to the channel frequency response matrix, the two -dimensional water-filling power allocation algorithm for MIMO-OFDM systems is proposed based on the Lagrange method, and the corresponding capacity analysis formula is derived. Simulation results s how that the capacity of a MIMO-OFDM system with the proposed two-dimensional water-filling power allocation is greater than that of the system with uniform power allocation and also greater than that of the system with the one-dimensional water-filling power allocation .