Interfacial properties of four kinds of natural fibers (et. ramie, jute, sisal and kenaf) reinforced phenolic resin were studied by single fiber pull-out test and short beam shear test. Effect of fiber surface treatments on the interfacial properties was evaluated. It showed that interlaminar shear strength (IFSS) was considerably improved after fiber surface treatments, especially after the silane treatment. Concerns about the poor moisture resistance of natural fibers, effects of fiber surface treatments on the moisture absorption behavior of natural fibers were also investigated by gravimetric methods. The results showed that neither fiber surface treatments nor the environmental temperature has effect on the moisture absorption behavior of natural fibers.