Wheat gluten/glycerol-based materials were reinforced through natural fiber addition. Hemp and wood fiber addition increased both composite tensile strength and Young's modulus, but decreased elongation at break. Resulting materials exhibited lower water sensitivity. Fiber addition does not modify the protein aggregation, but resulted in an increase of the composite matrix glass transition temperature (T-g). This increase was attributed to the plasticizer migration from the matrix to the fibers, which increased the matrix Young's modulus. This migration called matrix deplasticization contributed to the improvement of the overall composite mechanical properties. The true reinforcing effect attributed to the fiber addition and the deplasticizing effect attributed to plasticizer migration were dissociated and quantified. At high fiber content, deplasticizing effect became significant and reduced material processability. This study suggested that the fiber lignin content is susceptible to influence both the water sensitivity and the matrix deplasticization.