A wireless power transfer (WPT) transmitter with a class-E power amplifier is presented in this work. In the target application of implantable biomedical systems, variations of load condition as well as coil separation have significant impact on received voltage, power transfer capability, and power transfer efficiency, demanding for large design margin. In the proposed design, we adopt an impedance compression network (ICN) to compress the resulting impedance variation and, therefore, stabilize the performance. Duty-cycle control (DCC) is introduced for further improvement. Implemented in a 0.18-μm CMOS process and operating at 13.56 MHz, the WPT system includes the proposed transmitter and a power receiver with a cross-coupled differential rectifier. With load resistance varying from 65 to 85 Ω, measurement results show the rectifier’s output voltage of more than 1.3 V and power of more than 26 mW as the coil distance varies from 9.5 to 32 mm, which corresponds to a k range of 0.038 to 0.22. The maximum output power is 48.8 mW at k of 0.075, and the maximum PTE is 45.6 % with PA drain efficiency of 81 % at k of 0.2.