We investigate the thermoelectric properties of GaN with p- and n-type doping by the first principles calculation and the semi-classical Boltzmann theory. We find that the power factors (Sacr) of p-type GaN (-3500 μW/mK2) is about twice that of the n-type (-1750 μW/mK2), which indicates the thermoelectric properties of p-type GaN would be better. Thermal conductivity of GaN crystal decreases rapidly as the temperature increases, but it is still too large for thermoelectric applications. The figure of merit (ZT) estimated at 1500 K is 0.134 for p-type GaN crystal and 0.062 for the n-type.