  APE smearing and overlap-Dirac operator are combined to filter QCD vacuum
configurations. The results obtained from overlap fermions and improved 5Li
cooling are compared, both of them exhibit structures of dilute liquid of
instanton. Finally the overlap fermions, improved 5Li cooling and APE smearing
are combined to calculate the topological charge and identify the structure of
QCD vacuum. The results suggest dilute liquid of instanton dominance of
topological charge fluctuations in quenched lattice QCD.
