  We have utilized the concept of valon model to calculate the spin structure
functions of proton, neutron and deuteron. The valon structure itself is
universal and arises from the perturbative dressing of the valence quark in
QCD. Our results agree rather well with all the relevant experimental data on
$g_{1}^{p, n, d}$ and $g_{A}/g_{v}$, and suggests that the sea quark
contribution to the spin of proton is consistent with zero. It also reveals
that while the total quark contribution to the spin of valon is almost constant
at $Q^{2}>=1$ the gluon contribution grows with the increase of $Q^2$ and hence
requiring a sizable negative orbital angular momentum component $L_z$. This
component along with the singlet and non-singlet parts are calculated in the
Next-to-Leading order in QCD. We speculate that gluon contribution to the spin
content of the proton is about 60% for all $Q^2$ values. Finally, we show that
the size of gluon polarization and hence, $L_{z}$, is sensitive to the initial
scale$Q_{0}^{2}$.
