Next-to-Leading Order Description of Nucleon Structure Function In Valon Model

We have improved and examined the applicability of the valon model where the structure of any hadron is determined by the structure of its constituent quarks. Nucleon structure functions are calculated within this model in the Next-to-Leading order. The results compare well with the experimental data. The model handles the bound state problem and the calculations show a flat or almost flat behavior for $F_{2}$ which sets in at some region of $x\leq 10^{-5}$at fixed $Q^{2}$. The emergence of this behavior is a consequence of the model and was not put in a priori as a theoretical guess. It seems that such a flatness can be inferred from HERA data, although, not completely confirmed yet. A set of parton distributions are given and their evolutions are tested. Some qualitative implications of the model for the spin structure of the proton is discussed.