  We investigate the figure of merit of a quantum dot (QD) in the Coulomb
blockade regime. It is found that the figure of merit $ZT$ may be quite high if
only single energy level in the QD is considered. On the other hand, with two
or multi energy levels in the QD and without the Coulomb interaction, the $ZT$
is strongly suppressed by the bipolar effect due to small level spacing.
However, in the presence of the Coulomb interaction, the effective level
spacing is enlarged and the bipolar effect is weakened, resulting in $ZT$ to be
considerably high. Thus, it is more likely to find a high efficient
thermoelectric QDs with large Coulomb interaction. By using the parameters for
a typical QD, the $ZT$ can reach over 5.
