  We investigate the non-perturbative effect in the nucleon structure function
and the Gottfried sum by using a non-perturbative quark propagator with lowest
dimensional condensate contributions from the QCD vacuum. It is shown that the
non-perturbative effect modifies the conventional quark-parton model formula of
the nucleon structure function at finite $Q^2$ and suggests a non-trivial $Q^2$
dependence in the Gottfried sum.
