  We present the latest results on event-by-event fluctuations of charged
particle multiplicity in Au+Au and Cu+Cu collisions at $\sqrt{s_{NN}} = 200$
GeV and 62.4 GeV measured by PHENIX experiment at RHIC. The two particle
correlation length to discuss order parameters, which can be extracted from
scale dependence of the fluctuations, is supposed to be sensitive to the
critical points of QCD phase transition. The obtained correlation length with
respect to the pseudo rapidity gap indicates the power law behavior as a
function of the number of participant nucleons for Au+Au collisions at
$\sqrt{s_{NN}} = 200$ GeV.
