This paper presents the first fully integrated 24-GHz phased-array transmitter designed using 0.18- m CMOS transistors. The four-element array includes four on-chip CMOS power amplifiers, with outputs matched to 50 , that are each capable of generating up to 14.5 dBm of output power at 24 GHz. The heterodyne transmitter has a two-step quadrature up-conversion architecture with local oscillator (LO) frequencies of 4.8 and 19.2 GHz, which are generated by an on-chip frequency synthesizer. Four-bit LO path phase shifting is implemented in each element at 19.2 GHz, and the transmitter achieves a peak-to-null ratio of 23 dB with raw beam-steering resolution of 7 for radiation normal to the array. The transmitter can support data rates of 500 Mb/s on each channel (with BPSK modulation) and occupies 6.8 mm 2.1 mm of die area.