Abstract: In this paper,one novel frequency tuning circuit is presented.The bias of switch and distortion,which affect the performance of antenna,are considered in the design.One dual band antenna which shift between 850MHz/1800MHz and 900MHz/1900MHz is designed and tested.The antenna is mounted on one PCB whose dimension is similar as mobile phone.The tuning circuit consists of the low pass filter,DC block and one piece of Pin-diode.The test result shows that this antenna has high radiation efficiency and low distortion.