Recently, there are rapid developments in Wireless Local Area Networks (WLANs). To adapt to various WLAN environments, a WLAN antenna should be capable of operating at multiple frequency bands. For instance, these frequency bands may cover 2.400-2.484GHz (specified by IEEE 802.11b/g) and 5.150-5.350/5.725-5.825GHz (specified by IEEE 802.11a). Some monopole antennas for single-band are not suitable for this application. Most of the monopole antennas reported in the literature, such as the various lengths of two monopoles antenna rectangular microstrip patch antenna with a rectangular notch and CPW-fed circular slot antenna with a slit back-patch etc., are either large in antenna size or narrow in bandwidth.