A portable electronic device including an ultrasound transmitter, an ultrasound receiver, and a processing unit is provided. The ultrasound transmitter sends ultrasonic signals, while the ultrasound receiver receives reflected ultrasonic signals from an object. The ultrasound transmitter and the ultrasound receiver are disposed to form a reference axis. The processing unit processes the reflected ultrasonic signals to obtain a time-frequency distribution thereof, and determines a 1D gesture corresponding to projection loci of movements of the object on the reference axis according to the time-frequency distribution.