Surface treatment proves to be an effective way to enhance the mechanical properties of stainless steel fiber (SSF)-filled polymer composites, a candidate for shielding electromagnetic interference applications. To reveal the details of interfacial interaction in polypropylene (PP)-based composites, micro- and macromechanical performance of the materials as well as surface chemical features of the fibers extracted from composites were characterized. The results suggest that a chemical bond was formed between the SSF surface and modified PP (i.e., PP grafted maleic anhydride copolymer, PP-g-MAH)in addition to van der Waals forces, which accounts for the improved interfacial adhesion in the SSF/PP-g-MAH/PP system.