A folded loop antenna having a very compact size of ${\bf 5} \times {\bf 8} \times {\bf 60}\;{\bf m}{{\bf m}^{\bf 3}}$ with multiband operation for ultra-thin smartphone applications is proposed. The antenna generates four resonant modes, the traditional ${\bf 0}.{\bf 5}\hbox{-}{{\lambda }}$, ${\bf 1}\hbox{-}{{\lambda }}$, and ${\bf 1}.{\bf 5}\hbox{-}{{\lambda }}$ resonant modes and an extra higher-order ${\bf 2}\hbox{-}{{\lambda }}$ resonant mode, to cover the GSM850, GSM900, DCS1800, PCS1900, UMTS, TD-SCDMA, LTE2300, and WLAN systems. The extra ${\bf 2}\hbox{-}{ {\lambda }}$ resonant mode is generated using a certain distance between the feed point and the shorting point on the ground plane. The antenna is studied using simulation and measurement. To study the antenna in a more realistic situation, a standard metallic USB connector is placed inside the loop radiator. Results show that the USB connector has little effects on the antenna performance.