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We present universal BER performance ordering for different antenna sizes in Multiple-Input Multiple-Output (MIMO) wireless systems with zero-forcing (ZF) equalization, which hold for all SNR. We first show that when the number of transmit antennas is fixed, BER of each symbol degrades with a decrease in the number of receive antennas even if the received SNR is kept constant. Then, we prove that when the number of receive antennas is fixed, the average BER improves with a decrease in the number of transmit antennas, which shows the tradeoff between BER and bandwidth efficiency in MIMO with ZF equalization. Furthermore, discussions on the relation of BER with the same change in the number of receive and transmit antennas show that there is no universal order in BER for any particular channel. These highlight the advantage and the limit of MIMO with ZF equalization.