  I use a direct method to extract the longitudinal structure function in the
next-to-leading order approximation with respect to the number of active flavor
from the parametrization of parton distributions. The contribution of charm and
bottom quarks corresponding to the gluon distributions (i.e.,
$G_{n_{f}=3}(x,Q^{2})$ and $G_{n_{f}=5}(x,Q^{2})$) is considered. I compare the
obtained longitudinal structure function at $n_{f}=4$ with the H1 data
[Eur.Phys.J.C{\bf74}, 2814(2014) and Eur.Phys.J.C{\bf71}, 1579 (2011)] and the
result L.P.Kaptari et al.[ Phys.Rev.D{\bf 99}, 096019(2019)] which is based on
the Mellin transforms. These calculations compared with the results
  from CT18 [Phys.Rev.D103, 014013(2021)] parametrization model. The nonlinear
effects on the gluon distribution improve the behavior of the longitudinal
structure function in comparison with the H1 data and CT18 at low values of
$Q^{2}$.
