High-speed digital-to-analog converter (DAC) is key component in instrument and automatic test equipment, radar and ultra-wideband (UWB) systems. In this paper, a two-channel 1.2 GSps, 8 bit RF DAC for Multi-Nyquist applications in 1 µm GaAs Technology is presented. Combining mode select circuit with synchronous latch simplifies design of the current source and layout of DAC core circuit. Measurement results demonstrate that the Differential Nonlinearity (DNL) is within ±0.15 LSB, and the Integral Nonlinearity (INL) is within ±0.4 LSB. For the normal mode, the Spurious Free Dynamic range (SFDR) is larger than 40 dB; for the mixing mode, output bandwidth is up to 1.8 GHz, and the SFDR is larger than 30 dB. Under a supply voltage of 5 Volts, the output swing is 1.1 Vpp, and the total power consumption is 1.7 Walts for both channels working.