  After introducing the connection between the black hole horizon and the
cosmological horizon, we discuss the thermodynamic properties of
Reissner-Nordstrom-de Sitter (RN-dS) spacetime. We present the condition under
which RN-dS black hole can exist. Employing Ehrenfest' classification we
conclude that the phase transition of RN-dS black hole is the second-order one.
The position of the phase transition point is irrelevant to the electric charge
of the system. It only depends on the ratio of the black hole horizon and the
cosmological horizon.
