In MIMO-OFDM system which allows high data rate communications in order to increase system performance channel state information (CSI) is needed. CSI is determined by using channel estimation algorithms and then channel's dispersive effects are reduced CSI. In this study, the effect of the pilot density, velocity and channel's frequency selectivity on the performance of a MIMO-OFDM system was investigated. Computer simulations were used for this purpose and corresponding system performances are given in terms of bit error rate. In the simulations, pilot based block and comb type pilot arrangement, two ITU channels, 10Hz and 200Hz Doppler frequencies, %6 and %25 pilot densities were used. It is seen from the results; comb type pilot arrangement doesn't always yield the best system performance. The pilot arrangement that yields best system performance depends on pilot density, channel's time and frequency selectivity.