The thermoelectric properties of the Bi_1−xNa_xCuSeO (0.0 ≤ x ≤ 0.02) system have been investigated in the temperature range 300–923 K. Na doping significantly increased the carrier concentration to ∼0.92 × 10²⁰ cm⁻³ at the doping amount of x = 0.02. Furthermore, a relatively high carrier mobility and a slight Seebeck coefficient enhancement was seen, thus resulting in a high power factor of 8.0 μW cm⁻¹ K⁻² at room temperature. Coupled with a low thermal conductivity reduced by point defects scattering, this leads to a ZT of 0.91 at 923 K for Bi_0.985Na_0.015CuSeO which is nearly twice the value observed in pristine BiCuSeO.