In this paper,an ultra wide band fractal slot antenna with low radar cross section (RCS) has been designed.The Koch island with 90°extrusive angles has been implemented through 3 fractal iterations on the base of the square slot, the -10 dB impedance bandwidth range becomes 2.8 ~13.7 GHz from 3.0 ~13.1 GHz of the initial square slot antenna.The simulation and measured results show that the antenna exhibits relatively symmetrical pattern at the frequency of 3 GHz and 8 GHz. Furthermore,it has more stable gain and lower RCS compared with the square slot antenna in the whole frequency band.It can be applied to the occasion of the UWB antenna requiring of low RCS.