  We report the measurements of thermopower, electrical resistivity and thermal
conductivity in a complex cobalt oxide Li$_{0.48}$Na$_{0.35}$CoO$_{2}$, whose
crystal structure can be viewed as an intergrowth of the O3 phase of
Li$_{x}$CoO$_{2}$ and the P2 phase of Na$_{y}$CoO$_{2}$ along the c axis. The
compound shows large room-temperature thermopower of $\sim$180 $\mu$V/K, which
is substantially higher than those of Li$_{x}$CoO$_{2}$ and Na$_{y}$CoO$_{2}$.
The figure of merit for the polycrystalline sample increases rapidly with
increasing temperature, and it achieves nearly 10$^{-4}$ K$^{-1}$ at 300 K,
suggesting that Li$_{x}$Na$_{y}$CoO$_{2}$ system is a promising candidate for
thermoelectric applications.
