A handheld device and a planar antenna thereof are provided. The planar antenna comprises a radiator having a feeding point, a first short point and a second short point. The feeding point is coupled to a circuit board of the handheld device so that the handheld device transmits and receives a RF (radio frequency) signal through the radiator. The first short point is coupled to a ground of the circuit board so as to be grounded. A control element is disposed on the handheld device or the planar antenna in order to control the second short point to be selectively electrically coupled to the ground so that the planar antenna can operate at two different central frequencies. Furthermore, the planar antenna can operate at multiple central frequencies by changing a position of the second short point contacted to the radiator.