The effects of the grain size, lattice strain, and microstructure on the trapped field properties of ball-milled MgB 2 bulks were studied. A decrease in the in-plane lattice parameters and transition temperature and an increase in the in-plane x-ray diffraction peak widths occurred with milling as a result of grain refinement and carbon substitution. Microstructural analysis revealed that sub-micron-size fine MgB 2 grains were well connected, resulting in increased grain boundary density. The trapped field of the magnetized bulk samples exhibited a large improvement with milling and reached 3.72 T at 5 K, which is the highest among MgB 2 bulks prepared using pressureless synthesis. Excess milling resulted in degradation of the trapped field because of T c reduction. The increased grain boundary density and electron scattering are considered to contribute to the high trapped field through quantitatively and qualitatively enhancing the grain boundary flux pinning.