This paper shows a performance evaluation for ultra wideband (UWB)-based wireless body area networks (WBANs) through experiments by using a prototype. Specifications on the physical (PHY) and media access control (MAC) of the prototype are based on the draft standard for IEEE802.15.6. On the PHY, the prototpe uses UWB that its center frequency is 4.096 GHz with bandwidth of around 700 MHz. The MAC layer is based on a superframe structure comprising time slots assigned to a beacon slot, slots for contention access period (CAP), or slots for contention-free period (CFP). Experiments have been conducted with mounting the devices on human body in an anechoic chamber. Over-the-air experiments have been conducted when multiple nodes connecting to a single hub device send packets simultaneously. Through experiment results, it is shown that the number of slots allocated to CAP is crucial in order to maintain packet transmission successfully.