  A new formalism for thermodynamics of AdS black holes called the {\em
restricted phase space thermodynamics} (RPST) is proposed. The construction is
based on top of Visser's holographic thermodynamics, but with the AdS radius
fixed as a constant. Thus the RPST is free of the $(P,V)$ variables but
inherits the central charge and chemical potential as a new pair of conjugate
thermodynamic variables. In this formalism, the Euler relation and the
Gibbs-Duhem equation hold simultaneously with the first law of black hole
thermodynamics, which guarantee the appropriate homogeneous behaviors for the
black hole mass and the intensive variables. The formalism is checked in detail
in the example case of 4-dimensional RN-AdS black hole in Einstein-Maxwell
theory, in which some interesting thermodynamic behaviors are revealed.
