The aim of the present study was to investigate the effects of fiber content and then silane treatment on the mechanical performance of the natural fiber composites of recycled acrylonitrile–butadiene–styrene (ABS) provided by the automotive sector. Wood and palmyra fibers were used as fillers in 10% and 20% fiber content composites. The fibers were treated with N-(2-Aminoethyl)-3-aminopropyltrimethoxysilane to improve the interfacial adhesion between fibers and polymer matrices. The mechanical properties of the composites were determined by tensile and impact tests. Morphological analysis was later performed using a scanning electron microscope (SEM). According to the experiment results, the tensile and impact strength of both wood and palmyra fibers increase after silane treatment. However, for the low-wood-fiber-content composite, the tensile and impact strength decrease after silane treatment due to the presence of an excess amount of silane relative to fiber content. The addition of wood and palmyra fibers significantly improved the tensile modulus of composite material and further increases slightly after silane treatment. Finally, SEM analysis shows a homogenous mix of fibers and polymer matrices with fewer voids after silane treatment, thereby improving interfacial adhesion.