This paper investigates the effect of time-domain (TD) and frequency-domain (FD) interpolation on the performance of a Multi-Band (MB) Orthogonal Frequency Division Multiplexing (OFDM) Ultra-Wideband (UWB) system. We introduce a FD interpolator implemented by a radix-8 512-point IFFT architecture for applications on MB-OFDM UWB transmitters. For the specific application where the interpolation factor is fixed to four, the FD interpolator outperforms the TD interpolator implemented with digital low-pass FIR filters in terms of computational complexity. On the other hand simulation results show that FD implementation degrades the overall system performance for certain UWB channels.