An effective and accurate model is presented for co-simulation of circuits and UWB antennas in the time-domain, which can he used in SPICE simulators. An arbitrary UWB antenna is represented by cascaded ideal transmission fines characterized by S-parameter data. Using this model, we can efficiently simulate the entire system of circuits and UWB antennas, together, for transient analysis. Numerical analysis and computer simulations show this model is highly accurate, compact, stable and derivable.