With the higher and higher of transmitting rate, the signal bandwidth iswider and wider, and the

p ractical channel p resents the frequency selective fading character. In this paper, a multi - input multi -

output (M IMO) channelmodel under frequency selective fading is constructed according to the character of

frequency selective fading channel. The frequency and time domain methods are used to discussM IMO

channel capacity due to the known and unknown channel state information under the frequency selective fa2

ding. The simulation results show thatM IMO system can imp rove the channel capacity remarkably but no

exchanging signal bandwidth under the circumstances of the same launching power and transmitting band2

width, and the numbers of antennas and the quantity of SNR have the different effect on the channel capaci2

ty. The conclusion offers the basis of how to imp rove M IMO channel capacity under the circumstances of

frequency selective fading.