In this paper, a frequency reconfigurable microstrip slot antenna for ultra-wideband applications with switchable band-notch functions is designed and manufactured. To generate a wide usable fractional bandwidth of 3 to over 13.6 GHz, square patch transforms to a fork-shaped patch, also two triangular-shaped strips have been added at above sections of rectangular slot on the ground plane. Band rejections are obtained by adding two inverted L-shaped strips in the fork-shaped patch and one L-shaped defected ground structure. Moreover, by implementation of two PIN diodes within the antenna structure, four switchable frequency responses are achieved. The designed antenna has simple structure and a small size of 20 × 20 × 0.8 mm3 while providing band rejection characteristics at WiMAX (Worldwide Interoperability for Microwave Access) and WLAN (Wireless Local Area Networks) systems.