A static magnetic field has great effect on the solidification structure of monotectic alloys, so the rapid directional solidification in a static magnetic field has great potentials in the manufacturing of monotectic alloys. But up to date little is known about the details of the effect of the magnetic field on the microstructure development during liquid-liquid decomposition. Directional solidification experiments with Al-Pb monotectic alloys in a static magnetic field were carried out, and the effects of the field intensity and the solidification velocity on the solidification microstructure were investigated. Samples with well-dispersed microstructure were obtained. A model was proposed to describe the microstructure evolution in a monotectic alloy solidified in a static magnetic field, which was verified by comparing with experimental results and then applied to investigate the microstructure formation in the unidirectionally solidified Al-Pb alloys. The numerical results indicate that the convective flow of the melt is very strong during the liquid-liquid phase transformation, and leads to a nonuniform distributions of the nucleation rate, number density, average radius and volume fraction of the minority phase droplets along the radial direction of the sample, resulting in serious phase segregation in solidified structure. A static magnetic field can effectively suppress the convective flow and causes a more uniform distributions of the nucleation rate, number density, average radius and volume fraction of the minority phase droplets. The application of a static magnetic field is favorable for the formation of a well dispersed microstructure. The lower the Pb content of the alloy and the higher the solidification velocity, the lower the intensity of the magnetic field needed for obtaining a sample with homogeneous distribution of the minority phase particles.