  The van der Waals (VDW) equation of state describes a thermal equilibrium in
system of particles, where both repulsive and attractive interactions between
them are included. This equation predicts an existence of the 1st order
liquid-gas phase transition and the critical point. The standard form of the
VDW equation is given by the pressure function in the canonical ensemble (CE)
with a fixed number of particles. In the present paper the VDW equation is
transformed to the grand canonical ensemble (GCE). We argue that this procedure
can be useful for new physical applications. Particularly, the fluctuations of
number of particles, which are absent in the CE, can be studied in the GCE. For
the VDW equation of state in the GCE the particle number fluctuations are
calculated for the whole phase diagram, both outside and inside the liquid-gas
mixed phase region. It is shown that the scaled variance of these fluctuations
remains finite within the mixed phase and goes to infinity at the critical
point. The GCE formulation of the VDW equation of state can be also an
important step for its application to a statistical description of hadronic
systems, where numbers of different particle species are usually not conserved.
