This paper presents a novel ASK demodulator for passive UHF RFID (Radio Frequency Identification) tag, which can detect minimum 10% modulation index over a RF power range of 30dB. Compared with conventional nonlinear voltage and linear current demodulations, the detection sensitivity over a wider range is increased by adopting an innovative divisional linear voltage demodulation with ultra low power consumption. The demodulator is implemented in UMC 0.18µm mixed-mode CMOS technology. The power consumption is 3.95µW and it can be reduced to 3.08µW when the modulation index is large.