  The charm quark structure function $F^c_2$ and the longitudinal structure
function $F_l^p$ are directly sensitive to the gluon content of proton and
therefore are crucial in understanding of proton structure function, in
particular at low momentum transfer $Q^2$ and low Bjorken x. In the framework
of perturbative QCD the charm structure function is calculated in the leading
order (LO) and the proton structure function is investigated in the next
leading order (NLO) at small x region. The valence quark distribution is
obtained from the relativistic quark-exchange model. The calculated
$F_2^c(x,Q^2)$, $F_2^p(x,Q^2)$ and $F_L^p(x,Q^2)$, are compared with the
present available experimental data.
