A novel ultrawideband planar slot antenna is presented. In this design, one semicircular structure is etched from the ground plane and the end of microstrip-fed structure also adopts the same structure with another dimension. The simulated and measured results demonstrate that this antenna can provide at least 11:1 impedance bandwidth (VSWR≤2) and good radiation characteristics. Compared with other slot antennas, it is not only compact in structure, but also maintains comparable performances. Therefore, this antenna has some application prospect in wireless communication systems.