The time-domain channel estimator is compared with the frequency-domain channel estimator when multiple-input-multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) is employed. The time-domain maximum likelihood channel estimation (TMLE) and its mean square error (MSE) are derived assuming the knowledge of the maximum channel length. If we use only one OFDM symbol for the estimation, the preamble sequence of each transmit antenna uses a different set of subcarriers in MIMO-OFDM systems. As a result, the MSE of the frequency-domain least square channel estimation (FLSE) is derived assuming that linear interpolation is employed, numerical results show that the MSE of TMLE is smaller than that of FLSE and that the performance difference increases as the number of transmit antenna increases.