The aim of this work is to fabricate and investigate nickel nanowires (NiNWs) as a novel magnetorheological material and determine how the aspect ratio of these magnetic particles influences its magnetic properties. The latest methods for synthesizing NiNWs and nickel nanospheres (NiNSs) are presented and the corresponding magnetorheological fluids (MRF) are obtained. Materials were characterized so that the properties of NiNWs could be compared to NiNSs. As different size NiNWs were fabricated, their saturation magnetization values increased as the size increased. Moreover, MRF containing NiNWs processed shear stress 15 times as strong as the one with the same volume of NiNSs, although the saturation magnetization of NiNWs was smaller than NiNSs. MRF containing magnetic particles with more saturation magnetization and smaller coercivity usually has a stronger MR effect. Our result is interesting, and further finite element simulations were utilized to analyze the possible mechanisms. The simulation indicated that the large aspect ratio of NiNWs helped to align the particles into columns and also caused the magnetized direction of particles to deviate from the direction of the applied field, thus restoring the torque and achieving a large shear stress. Furthermore, MRF with higher fraction of NiNWs has a more stable suspension, and NiNWs disperse much better than NiNSs with the same volume.