  Recently, it has been shown in Ref.[1] that the string thermodynamic
fluctuation may lead to a scale invariant spectrum of scalar metric
perturbation. However, its realization is still in study. In this note we
suppose that the correlation length of metric perturbation, which is
proportional to the sound speed, might be nearly divergent at the critical
point of phase transition. In this case we find that the string gas mechanism
responsible for the generation of primordial perturbation may be applied well.
