A dual-band single-layer single-feed dual-band U-slot antenna, with linear polarization and single/dual-beam radiation pattern, is proposed for WLAN [2.40-2.4835 GHz and 5.15-5.35 GHz (indoor)] systems. The impedance bandwidths (|S11| < −10 dB) of 7.3% and 12.7% are achieved at central frequencies 2.40 and 5.25 GHz, respectively. The lower band radiation pattern is a single broadside beam and the peak gain is 6.3 dBi at 2.40 GHz. The higher band radiation pattern is dual-beam and the peak gains are 7.7 dBi and 7.2 dBi for different beams at 5.25 GHz. The simulated results and the measured ones are in good agreement.