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Bulk MgB2 with SiO2 nanoparticles added has been synthesized using a simple solid-state reaction route. The lattice constant in the c direction increases with additive content due to a small amount of Si being doped into the lattice of the MgB2; however, Tc is almost fixed at 37.2 K. The addition of SiO2 nanoparticles also improves the Jc–H and Hirr–T characteristics of MgB2 when the additive content is lower than 7%. At 20 K and 1 T, Jc for the sample with 7% additive content reaches 2.5 × 105 A cm−2. Microstructural analysis reveals that a high density of MgSi2 nanoparticles (10–50 nm) exists inside the MgB2 grains, leading to the formation of a nanocomposite superconductor.