This paper presents a mixed mode rectifier circuit operating at 13.56 MHz for wireless power transmission system to rectify the AC signal and power up the implantable medical device. The proposed design includes both voltage and current mode operation that covers a wide range of coupling ratios between the coils. The circuit is designed in 180 nm CMOS technology. Extracted simulations show that the mixed mode design charges the load with the maximum efficiency 72.4% for the voltage mode, and 38.6% for the current mode at the operating frequency 13.56 MHz for both modes. The voltage and current modes are beneficial for high and low coupling ratios, respectively. Operation under different modes extends the usable coupling range in power transmission.