A 0.18μm CMOS fully integrated cascode Class-E power amplifier operating in 2.5GHz/3.5GHz/5.2GHz frequency bands for polar transmitters has been proposed. The tri-band amplification is achieved by adaptation of the common-gate transistor size and the matching networks. The phase distortion from supply modulation of the Class-E is compensated by controlling the common-gate transistor gate voltage and a compensative capacitor. Simulation results show that the maximum power-added efficiency (PAE) of 30.4%, drain efficiency of 34% and output power of 18.8dBm from 2.8V supply can be achieved. The phase distortion can be compensated from 34.4° to 3.1° under supply voltage of 0.5V to 2.8V. A system co-simulation has been established for relative constellation error (RCE) evaluation and it reveals that the RCE can be improved from -23.3dB to -31.4dB in 3.5GHz frequency band.