  We prepared the brownmillerite oxide Ca_{2-y}La_yCo_{2-x}Al_xO_5, and found
that it was an n-type conductor. The thermopower and the resistivity of the
single crystal are -90 microV/K and 68 mOhm cm along the ab direction at 440 K,
which suggest relatively good thermoelectrical properties, compared with other
transition-metal oxides. Their temperature dependences are of activation type,
and the activation energies are 0.2 eV for the resistivity and 0.04 eV for the
thermopower. These energies differ by one order in magnitude, which implies
that a polaron dominates the charge transport. A sign of the thermopower of the
polycrystals changes from negative to positive at 500 K, indicating that holes
are excited thermally to decrease the magnitude of thermopower.
