A pulsed magnetic field (PMF) was introduced into the solidification of pure magnesium and the effect of structure refinement is investigated. The results show that remarkable microstructural refinement is achieved when the pulsed magnetic field is applied. The morphology is characterized as the columnar grains at the periphery and equiaxed grains at the core. The average grain size in the center of ingot is refined to 260m at the optimal processing parameter (5Hz, 200V). The pulsed magnetic field increases melt convection during solidification, as a result, the stress fractures the dendrites inwards from the mould wall, and the fractured dendrites act as nuclei, so the big columnar grains are prohibited.