Abstract: For the channel estimation of MIMO-OFDM (multiple-input multiple-output-orthogonal frequency division multiplexing) systems based on IEEE 802.11a standard, a least square (LS) channel estimation scheme is proposed in time domain. The idea of this scheme is mainly to estimate the time-domain channel responses between transmitting and receiving antennas in accordance to the time-domain signals between transmitting and receiving ends. To make sure that the mean square error (MSE) is minimum in the LS channel estimation, an optimal scheme of pilot sequence is put forward, which requires less training symbols than conventional schemes and the number of those symbols will not increase with the increasing number of antennas. Moreover, the number of transmitting antennas is unlimited. In the scheme proposed all the pilot tones are required to be equipowered and the phase shift should be mutually orthogonal among the pilot sequences of different transmitting antennas. Simulation results show the effectiveness of the scheme proposed.