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 of the relevant experimental data on g₁^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 a valon, ΔΣ_valon, is almost constant at Q² ≥ 1 the gluon contribution grows with the increase of Q² 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 Q² values. Finally, we show that the size of gluon polarization and hence, L_z, is sensitive to the initial scale Q₀².