  We have investigated the thermodynamical properties of dark energy. Assuming
that the dark energy temperature $T\sim a^{-n}$ and considering that the volume
of the Universe enveloped by the apparent horizon relates to the temperature,
we have derived the dark energy entropy. For dark energy with constant equation
of state $w>-1$ and the generalized Chaplygin gas, the derived entropy can be
positive and satisfy the entropy bound. The total entropy, including those of
dark energy, the thermal radiation and the apparent horizon, satisfies the
generalized second law of thermodynamics. However, for the phantom with
constant equation of state, the positivity of entropy, the entropy bound, and
the generalized second law cannot be satisfied simultaneously.
