: The rapid progress and growing demand in the provision of small portable cellular/non-cellular devices, capable of exchanging data, sending voice and video and connecting to the internet, has increased the focus on designing small multi-band frequency antennas. In fact, Planar Inverted F Antenna (PIFA) is made based on one of the various techniques used to reduce the size of microstrip antennas which changes them from half wavelength structures to quarter wavelength structures. Simple structures and low manufacturing costs as well as their small sizes, has cause PIFAs to be widely used in all small wireless devices. In this thesis, a new way of designing quarter-band PIFA with central resonant frequencies of 1.2 GHz, 2.4 GHz, 3.7 GHz and 5.8 GHz has been presented. The antenna is linearly polarized in all its frequency bands and meets the requirements of wireless frequency bands. The proposed antenna consists of a main U-shaped radiator which is set on the RO3003 substrate, and two shorting walls which shorten the patch to the ground plane. A rectangular slot is added to the ground plane in order to improve the radiation pattern of the antenna at 2.4 GHz and to increase the bandwidth. Finally, considering the requirements of making an antenna, the substrate material is changed from RO3003 to Duroid 5880 which leads resonance frequencies to occur at 1.1 GHz, 2.42 GHz, 3.64 GHz and 5.1 GHz. Keywords: Microstrip Antenna, Planar Inverted-F Antenna, Slot, Improivng radiation pattern, Bandwidth, Multiband Frequency