In this paper, a novel low-profile planar monopole internal antenna for GSM/DCS/PCS/ IMT/UMTS/WLAN/ISM/LTE operation in the mobile phones is designed and developed. The proposed antenna is composed of multi-branches, F-shaped slots in the system ground plane and tapered feeding line which can improve the impedance matching at the feeding point. The antenna occupying a small area of 18.5 × 46 mm2 is placed on the top no-ground portion of the system circuit board, which makes it suitable for practical mobile applications. A prototype of the proposed antenna is fabricated and tested. In addition, the planar monopole antenna can provide two wide lower and upper bands to respectively cover the frequency range of 848–1152 MHz, and 1736–3000 MHz, for the GSM850/GSM900/DCS1800/PCS1900/IMT2000/UMTS2100/WLAN2400/ISM2450/LTE2300/LTE2500 operation in the mobile phone. Good radiation patterns and antenna peak gain for frequencies over the operating bands have been observed. The antenna is simulated and designed by using Ansoft HFSS and CST Microwave Studio. Details of the antenna design, results on the reflection coefficient, radiation characteristics, directivity, antenna gain, realized gain, and efficiency of the antenna are given and discussed. Finally, the specific absorption rate (SAR) of the proposed antenna placed at the bottom of the mobile phone is also investigated. The obtained SAR values meet the limit of 1.6 W/kg for the 1 g head tissue and 2.0 W/kg for the 10 g head tissue.