Recently the Ultra-WideBand (UWB) systems have expanded rapidly, since Federal Communications Commission (FCC) allocated the frequency band of 3.1 to 10.6GHz for the operation of devices with EIRP less than -41.3dBm/MHz. Besides the return loss and radiation pattern, dispersion characteristic of the antenna is one of the most important factors which should be considered in ultra-wideband applications. In this paper, dispersion behavior of two specific Vivaldi antennas has been fully investigated in both frequency and time domains. All simulations are carried out by CST MS software. Moreover, by fabricating the antennas, the simulation results are verified by experimental data.