A broadband and low-loss 1:4 transmission-line transformer (TLT) fabricated in 0.18m CMOS process is proposed. Using broadside-coupled and multiple-metal stacked transmission lines, the broadband impedance transformation is from 12.2±0.1 to 50Ω within a 1.2GHz bandwidth from 2.1 to 3.3GHz, and the minimum insertion loss is 1.0dB at 2.6GHz with a 3dB bandwidth of 180. In addition, a fully-integrated CMOS class-E power amplifier (PA) is designed to demonstrate the capability of the proposed 1:4 TLT, which is used as the output impedance transformer of the class-E PA. The maximum output power is 24.7dBm at 2.6GHz, where the power-added efficiency is 33.2 and the power gain is 13.2dB under 3.6V supply voltage. The class-E PA achieves broadband and flat output power of 24.6±0.2dBm from 2.4 to 3.5GHz.