The nanosized magnetite particles were synthesized by means of a coprecipitation method and used to prepare the aqueous ferrofluids with various volume fractions. The magnetorheological properties of the aqueous ferrofluids have been investigated as a function of magnetic field. Under steady-state shear, the apparent viscosity for all of the ferrofluids exhibited a shear thinning behavior. But the Bingham model was invalid since the ferrofluids did not show a yield stress before they began to flow. However, the shear stress increased linearly with shear rate after a critical shear rate, showing a Bingham-like behavior. A Bingham-like yield stress, which was obtained by extrapolating the shear stress to the zero shear rate, increased with both the volume fraction of magnetic particles and the strength of magnetic field. The Bingham-like viscosity was approximately independent of the magnetic field at a given volume fraction of magnetic particles, but increased with the volume fraction under a given magnetic field. In the strain sweep experiment at an angular frequency of 10 rad/s, a transition from a gel-like state to a sol-like state was observed and a chain model has been proposed to qualitatively explain the mechanism of the transition. From the frequency sweep tests, it was found that there existed a plateau of storage modulus G′, which was independent of frequency but dependent on the volume fraction. A scaling law has been proposed to correlate the G′ plateau with the volume fraction.