This paper proposes a new I/Q imbalance estimation algorithm for multiband orthogonal frequency division multiplexing (MB-OFDM) based ultra-wideband (UWB) systems. The proposed algorithm is highly effective for the UWB channels with strong frequency selectivity in the presence of carrier frequency offset (CFO). In order to estimate I/Q imbalance, the 2nd-order moments of received signal are analyzed, and a metric function is derived considering CFO. The I/Q imbalance is estimated based on this function which can be sequentially calculated with only two preamble symbols per each sub-band in time-domain, which ensures immediate convergence and robustness to frequency selectivity. The performance of the algorithm is evaluated by simulations in the case of 480 Mbps transmission mode in WiMedia MB-OFDM system. The simulation results show that the proposed algorithm achieves about 4 dB SNR improvement over the existing methods at BER=10-5.