Provided is a thermoplastic resin composition, including (a) 100 parts by weight of a thermoplastic resin including 80-100% by weight of a base resin and 0-20% by weight of a reinforcing resin; (b) 2-60 parts by weight of linear carbon fibers having an average diameter of 1-15 μm; (c) 1-5 parts by weight of carbon nanofibrils having a BET specific surface area of 200-400 m2/g; (d) 1-15 parts by weight of carbon nanoplates; and (e) 1-25 parts by weight of metal powder, a method of preparing the thermoplastic resin composition, and an injection-molded article manufactured using the thermoplastic resin composition. The thermoplastic resin composition has excellent mechanical properties, e.g., impact strength, and also excellent conductivity, heat resistance, and electromagnetic wave shielding capacity, particularly high shielding efficiency against high-frequency electromagnetic waves, and thus can be used as automobile, electric, and electronic parts, and as a substitute for aluminum alloys and magnesium alloys.