  The vacuum properties of lattice QCD with staggered quarks are investigated
by an efficient simulation method. I present data for the quark condensate with
flavor number $N_f=0, ~ 1, ~ 2, ~ 3, ~ 4$ and many quark masses, including the
vacuum energy in the chiral limit. Obvious sea quark effects are observed in
some parameter space. I also describe a mechanism to understand this and a
formula relating the chiral condensate and zero modes.
