This paper presents a bandwidth-enhancing technique for the design of quad-band PIFAs (Planar inverted F Antenna). By using a simple parasitic metal strip, the bandwidth of a typical PIFA is considerably broadened. With the proposed technique, the bandwidths of all passbands can be easily adjusted without significantly affecting that of the others. In the paper, the proposed quad-band PIFA fabricated in the air substrate for GSM900/DCS1800 handset is first designed. Then a metal strip is placed along the PIFA, and a quad-band (GSM900/DCS1800/PCS1900/UMTS2100) antenna is obtained. The position of the parasitic strip is carefully studied and chosen, so the performance of the PIFA in GSM900 and DCS1800 bands is not affected by the strip. The measured bandwidths of the proposed antenna are 80 MHz (880960 MHz, S11 < 6 dB) in the lower band, and 475 MHz (16952170 MHz, S11 < 10 dB) in the upper band. Other results such as the simulated and measured reflection coefficient, simulated current distribution, and measured antenna patterns and gains are also presented.