This letter presents CMOS voltage controlled oscillator (VCO) circuit, as a part of Automatic Dependent Surveillance-Broadcast (ADS-B) transmitter, operating at the center frequency of 980 MHz, delivers highest output power with highest efficiency ever reported to the best of authors’ knowledge. The 0.713 mm × 0.638 mm VCO chip fabricated using 0.18 μm CMOS technology exhibits the single-ended peak output power of 1.4 dBm and the differential peak output power of 4.4 dBm at the frequency of 1.097 GHz, with the frequency tuning range from 980 MHz to 1.1 GHz, and phase noise of −125 dBc/Hz at 1 MHz offset centered at the frequency of 980 MHz, consuming 48-mW power from 1.8-V supply, thus showing state-of-the-art performances. This promising result shows a path toward realizing high-power single-chip CMOS ADS-B transmitter. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:2437–2441, 2015