In this paper, the design and nonlinear analysis of a dual-band mm-wave Doherty power amplifier is presented. The Doherty power amplifier has been designed to operate simultaneously at 24GHz for ISM band and 28 GHz for LMDS applications with 1 GHz bandwidth at each band, using Large-signal model of 0.15µm GaAs PHEMT transistor with 6 dB of output power back off at both frequencies. Analysis results show that DPA improves both efficiency and linearity in a wide power ranges for these two frequency bands.