The objective of this research was to study the effects of wood-surface roughness, adhesive viscosity and processing pressure on adhesion strength between soybean protein adhesive and wood, and to seek the relative importance of the individual factors in determining adhesion strength. Processing pressure was found to be the most important factor in determining adhesion strength. An optimum pressure, which was about 4.55 MPa in this research, is needed for development of a strong bond. A higher pressure resulted in reduced adhesion strength, possibly due to damage to the wood surface; a lower pressure also resulted in decreased adhesion strength because of the lack of bond formation. Adhesive viscosity had greater effect on adhesion strength than surface roughness. Contact angle, which was found to be mainly determined by adhesive viscosity and surface roughness, was a major factor controlling adhesive penetration. A smaller contact angle, resulting from lower viscosity and rougher surface, produced deeper penetration, while a larger contact angle, resulting from higher viscosity and smoother surface, produced shallower penetration. An optimum penetration is needed to enhance adhesion strength by developing a three-dimensional interactive zone at the interface. Too deep or too much penetration would result in 'dry-out' at the interface; less penetration would limit the formation of the three-dimensional zone at the interface. Both cases resulted in reduced adhesion strength. Contact angles ranging from 35 to 47 degrees provided the optimum penetration needed for good adhesion. The results of this research could be used as reference to determine optimum process parameters in plywood manufacturing when an aqueous based adhesive is used.