The effect of a static magnetic field on the solidification of monotectic alloy has attracted great attentions. But up to date little is known about the details of the magnetic field influences on the microstructure development during the solidification of a monotectic alloy. In this paper, rapid directional solidification experiments are carried out with Al-Pb alloys under a static magnetic field. The dependence of the solidification microstructure on the intensity of the magnetic field is investigated. The mechanism through which the magnetic field affects the microstructure formation is analyzed. It is indicated that the static magnetic field causes only a small decrease in the Marangoni migration velocity and the Stokes settlement velocity of the minority phase droplets during the liquid-liquid decomposition. Such a small change in the moving velocity of the droplets has only a negligible effect on the microstructure formation. The static magnetic field can suppress the convection efficiently. It enhances the spatial homogeneity of the liquid-liquid phase transformation along the radial direction of the sample, reduces the collision and coagulation frequency between the minority phase droplets and, therefore, causes a decrease in the largest and average sizes of the Pb-rich particles in the solidified sample as well as in the width of the particles size distribution. A static magnetic field is favorable for obtaining a monotectic alloy with well dispersed microstructure.