In order to absorb and rectify as efficiently as possible the radio frequency energy distributed in the sur- rounding environment，this paper proposes a dual-band ＲF energy harvesting system operating in GSM1800 and UMTS2100 frequency bands． The receiving antenna of this system adopts a planar broadband yagi antenna，which has compact structure，high measured average gain and stable directional radiation pattern in the working frequency bands． In the low input power circumstance from －20 dBm to －5 dBm，the rectifying circuit of this system can achieve good dual-band resonance at 1. 8GHz and 2. 1GHz． The maximum rectification efficiency can reach 45% in －5dBm． Finally，the overall system has been tested in the laboratory and ambience． The results show that the proposed dual-band system can achieve 45% ＲF-DC conversion efficiency in －5 dBm in the laboratory measure- ment，and the proposed system is qualified for harvesting ＲF energy and achieves a certain amount of DC power on the resistance load in ambient environment．