Antenna designs fabricated on thin flexible substrate material with thickness as thin as 45 μm  are presently receiving a lot of attention in the antenna industry. This is because such antenna with very thin substrate can be more sophisticated (bend and fold) and un-obstructive when integrated with the RF circuit. Furthermore, to accommodate the emerging trend for mobile devices such as ultra-thin notebook or mobile phone, lesser spaces are now given to the antenna engineer to insert the antennas, especially for WLAN applications. Therefore, rigid substrate material (such as FR4, Taconic, and Roger) for the fabrication of antennas will eventually be replaced by thin flexible substrate with a low-cost of approximately US $5/per sheet (dimension 250 × 230 mm2). Various dual-band WLAN antenna designs that fabricated on flexible substrate have been reported [1-3]. However, these antennas have demonstrated either a complicated fabricating process  or a large antenna size of up to 33 × 28 mm2 . In this paper, a compact size (11 × 7 mm2) planar antenna printed on a thin flexible substrate for dual-band operation in the 2.4/5.2 GHz WLAN application is proposed. Details of the flexible substrate are presented, and typical antenna performances such as return loss, radiation patterns, and gain variation are also discussed.