With induction of external magnetic fields, ultrafine Ni fibers with length up to 100 μm and diameter of about 250 nm were fabricated by a liquid phase reduction method under normal pressure. Then the obtained Ni fibers and commercial microsized Ni powder were used as filler to fabricate resin-based conductive composites, respectively. The electromagnetic shielding effectiveness (SE) of the resultant resin-based conductive composites was measured as a function of nickel content in the frequency range from 130 MHz to 1.5 GHz. Results indicate that compared with the composite with microsized Ni powder, the resin-based composite with ultrafine Ni fibers shows better electromagnetic shielding effectiveness. The composite with 33.3% (mass fraction) Ni fibers possesses an average SE of 15.7 dB.