Effects of treating cotton stalks with a coupling agent (maleic anhydride polypropylene (MAPP), polymeric methylene diphenyl diisocyanate (PMDI), or vinyltrimethoxysilane (A171)) were investigated relative to the mechanical and water resistance properties of cotton stalk–polypropylene film boards. The interfacial morphology was investigated by scanning electron microscopy (SEM). The results showed that the properties of the boards increased with the incorporation of MAPP or PMDI up to 2 wt%, or with A171 up to 3 wt%, but further increases in the coupling agent contents decreased the properties. Boards treated with 2 wt% PMDI exhibited optimum properties, with a 39.0% increase in modulus of rupture, a 38.2% increase in modulus of elasticity, a 68.4% increase in internal bond, and a 57.4% decrease in thickness swelling, compared with untreated boards. The SEM micrographs further confirmed an efficient fiber-film adhesion in the coupling-agent-treated boards.