The wideband characteristics of planar antennas with a wide radiating slot have been examined in this paper. A various resonance modes are generated in the wide slot and the wideband characteristics are enhanced by impedance match at each resonance frequencies. In this process, the geometry effects on the impedance match were extracted and it was found that a round corner rectangular slot antenna has more stable impedance match over a wide frequency range than a normal rectangular slot antenna. The round corner rectangular slot antenna is fabricated to verify the wideband characteristics and its measured bandwidth is almost 2 octaves(2.08 ㎓~8.25 ㎓) with VSWR$leq$2.