Two kinds of anisotropic cellulosic nanoparticles, cellulose nanocrystal (CNC) and nanofiber (CNF), were used as additives for the aqueous carbonyl iron particles (CIPs) based magnetorheological (MR) fluids, aiming at disclosing effect of nanocelluloses on the magneto-responsive behavior and stability of MR fluids. Both CNC and CNF can stabilize MR fluids, and improve their sensitivity to alterations of magnetic field strengths. Relative to electrostatic repulsion, physical hindrance is more important to the stability improvement. Thus, CNF is the better option because it has higher aspect ratio and surface charge. Moreover, the presence of cellulosic nanoparticles hinders coalescence of CIP chains to stronger columns, and hence the nanoparticles stabilized MR fluids show thixotropy evidently. At the comparative concentration (0.3 wt%), the presence of CNF even enhances the magneto-responsive levels because the percolated CNF network acts as stress amplifier. This work opens up a new door for applications of nanocelluloses in MR fluids.