A novel CPW-fed multiband planar monopole antenna is designed and investigated in this paper. The proposed antenna is fabricated on FR4 substrate and has a compact size of 26 × 45 × 1.6 mm3. A prototype is fabricated and then measured. The simulated and measured results exhibit that the operating bandwidth with a reflection coefficient of less than −10 dB is about 150 MHz (1700—1850MHz), 310 MHz (2.38—2.69 GHz), 520 MHz (3.23—3.75 GHz), and 820 MHz (5.09—5.91 GHz). These frequency ranges meet the bandwidth requirement for GSM 1800MHz, WLAN 2.4/5.2/5.8 GHz and WiMAX 2.5/3.5/5.5 GHz bands. Also, almost omnidirectional radiation patterns and peak gains of 2.1dBi at 1.8 GHz, 2.21dBi at 2.4 GHz, 2.8dBi at 3.5 GHz, and 4.98dBi at 5.5 GHz are obtained. The proposed antenna with small size, planar structure, good impedance bandwidth and omnidirectional radiation patterns is very suitable for GSM/ WLAN/ WiMAX applications.