The thermoelectric properties of intrinsic n-type β-Ga2O3 are evaluated by first-principles calculations combined with Boltzmann transport theory and relaxation time approximation. The electron mobility is predicted by considering polar optical phonon scattering in β-Ga2O3. A temperature power law of T-0.67 is obtained for the intrinsic electron mobility. Due to the ultra-wide band gap of 4.7-4.9 eV, β-Ga2O3 has a large Seebeck coefficient. As a result, a maximum power factor of 3.1 × 10-3 W m-1 K-2 is obtained at 1600 K. A clear anisotropy in lattice thermal conductivity is observed, with the highest thermal conductivity of 23.1 W m-1 K-1 at 300 K along the [010] direction, and a lower value of 13.2 and 12.2 W m-1 K-1 along the [001] and [100] directions, respectively. A high ZT value of 1.07 at 1600 K can be obtained at the optimal carrier concentration of 2.4 × 1019 cm-3, which is superior to that of most other oxides such as ZnO. In addition, the lattice thermal conductivity can be reduced by precisely adjusting the grain size, and the lattice thermal conductivity at 300 K (1600 K) can be reduced by 73% (39%) when the grain size is decreased to 10 nm. The excellent thermoelectric properties of β-Ga2O3 have promoted its potential application in the field of high temperature thermoelectric conversion.