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A T/R switch, fabricated using standard 0.25-μm CMOS process, for ultra wide-band (UWB) wireless communications is presented. The switch is designed based on the concept of synthetic transmission line, utilizing both CPW and CMOS transistors, to achieve not only an extremely wide bandwidth but also a linear phase response necessary for time-domain UWB applications. On-chip measurement is completed in both frequency and time domains. Frequency-domain measured results show insertion loss of 2.2-4.2 dB, isolation from 34-48 dB, and highly linear transmission phase from 0.45 MHz to 13 GHz. These results are quite consistent with the calculations. Particularly, the time-domain pulse measurement shows that the output pulses resemble closely the input pulses with very little reflection, demonstrating the switch's suitability for true time-domain UWB applications. The developed switch is ready to be integrated with other CMOS RFICs to form on-chip transceivers for various UWB applications.