Abstract: This paper studied the effect of silane coupling agents on the flexural properties and water resistance of HAC/PVA based macro defect free (MDF) composites. The chemical interaction between γ-methacryloxypropyl trimethoxy silane (A-174^TM) and monocalcium aluminate (CaAl₂O₄) was also studied using x-ray photoelectron spectroscopy (XPS). The SiOH group in A-174^TM reacted with M-OH group in the surface of CaAl₂O₄ to form M-O-Si- bond (M=Ca,Al) in a condensation reaction. A new Ca(2p_3/2) signature at 347.65ev or Ca(2p_1/2) signature at 350.85ev and a new Al(2p) signature at 73.90ev were associated with the formation of the new bond. Accordingly, the authors proposed the chemical reaction model of the silane cross-coupling agent at HAC/PVA interface and the mechanism of the silane cross coupling agent in increasing the strength and improving the water resistance of HAC/PVA based MDF composites.