This paper presents a 258-GHz CMOS transmitter with phase-shifting capability for phased-array systems targeting sixth-generation mobile network (6G). Phase shifters are inserted in local oscillator (LO) paths to limit the bandwidth required of the phase shifters. A power detector is used to detect and calibrate the phase difference between a pair of phase shifters in the two power-combined paths of the transmitter. It is fabricated using a 40-nm CMOS technology. A data rate of 16 Gb/s and output power of-6.8 dBm are achieved with quaternary phase-shift keying (QPSK). The calibration procedure of the phase shifters is verified by simulation.