This paper presents a low-power all-digital phase modulator (PM) pair to generate constant-envelope signals for LINC transmitters. To reduce the power overhead, an open-loop delay line based phase shifter with a continuous locking scheme is adopted for the PM design. This design is implemented by 90 nm CMOS technology with active area of 0.1892 mm 2. The PM provides 8-bit resolution with RMS error 0.33 at IF frequency 80 MHz, and the power consumption is only 885 uW with 1.0 V supply voltage. Considering a 64-QAM OFDM system, the EVM of 31.87 dB can be achieved by using the proposed PM pair.