Bulk samples of MgB2 were prepared with 5, 10, and 15 wt % Y2O3 nanoparticles, added using a simple solid-state reaction route. Transmission electron microscopy showed a fine nanostructure consisting of ∼3–5 nm YB4 nanoparticles embedded within MgB2 grains of ∼400 nm size. Compared to an undoped control sample, an improvement in the in-field critical current density JC was observed, most notably for 10% doping. At 4.2 K, the lower bound JC value was ∼2×105 A cm-2 at 2 T. At 20 K, the corresponding value was ∼8×104 A cm-2. Irreversibility fields were 11.5 T at 4.2 K and 5.5 T at 20 K. © 2002 American Institute of Physics.