Polyaniline nanofibers have been synthesized by simple chemical polymerization method without using any structure directing agents and organic solvents. The effects of various reaction conditions on the morphology and electromagnetic interference (EMI) shielding properties of the polyaniline nanofibers were investigated. The detailed study of various reaction conditions indicates that stable surfactant free, room temperature oxidation of aniline hydrochloride by ammonium persulfate at low oxidant to monomer ratio in acidic media yields self-assembled uniform polyaniline nanofibers. The nanofibers have excellent EMI shielding efficiency of about 71–77 dB in the frequency range of 8.2–18 GHz.