Bit error performance is investigated in this letter for orthogonal space-time block coded (STBC) multiband orthogonal frequency division multiplexing (MB-OFDM) ultra-wideband (UWB) systems. As the channel model considers the log-normal fading, the resultant signal to noise ratio (SNR) follows a chi-squared log-normal distribution. Based on this, the bit error probability is derived. Furthermore, simulation verifies this theoretical result.