This letter presents a dual band slot antenna with high gain for WLAN and WiMAX communication covering the IEEE802.11a/b and IEEE802.16d/e standards. The antenna consists of a microstrip fed slot with a complimentary stub on a single dielectric medium. A reflecting ground plane is employed to achieve a unidirectional radiation pattern with an average gain of 9.1 dBi. The feed of the antenna is optimized to achieve radiation efficiency above 95% in the WLAN and WiMAX frequency bands. The radiation properties of a prototype antenna were measured in a compact antenna range. Simulated and measured results (impedance bandwidth, gain and radiation patterns) are presented for the proposed antenna.