A miniaturized ultra-wideband antenna is designed in P band. The antenna is fed by a pentagon coupled feeding structure. An improved rectangular slot with saw-tooth edge is etched on the ground. The results of parameters show that the bandwidth of the designed antenna is 293.4~830.3 MHz with relative bandwidth 95% and the return loss is less than -10 dB. The size of the antenna is ( is the wavelength of the lowest frequency). The average gain is over 3dBi in the operating range and the antenna is a kind of omni antenna. The antenna is a planar construction and is simple in shape and easy in conformation. A sample antenna is fabricated and tested. The experimental results are in good agreement with the simulation results.