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