  We investigate the non-perturbative effects on the ratio $R=\sigma_L
/\sigma_T$ in lepton-nucleon deep inelastic scattering by taking into account
the lowest dimensional condensate contributions from the QCD vacuum. By
combining conventional perturbative QCD corrections and the Georgi- Politzer
target-mass effects with the non-perturbative effects from the QCD vacuum, we
give a good description of the $Q^2$ and $x$ dependences of $R$ in comparison
with the recent experimental data.
