Monotectic systems are a kind of extensive alloys. Many of them have great potentials for practical industry application. A lot of work has been carried out to study the solidification of monotectic alloys. But most of them focused on the microstructure formation during cooling a hyper-monotectic alloy through the miscibility gap in the liquid. Little work was done on the solidification behaviors of an alloy of monotectic composition. Directional solidification experiments were carried out with Al-Bi alloy of the monotectic composition (Al-3.4%Bi, mass fraction). The influences of the solidification velocity on the microstructure were investigated. The microstructure evolution during solidification was analyzed. The results indicate that a solute-rich layer forms in front of the solidification interface and the liquid-liquid decomposition occurs there. When the alloy is solidified at such a high velocity so that the minority phase droplets of all sizes in front of the solidification interface are migrating to the solidification interface, the size distribution of the minority phase particles shows only one peak. When the alloy is solidified at a relatively low velocity, the minority phase droplets within a certain size range may move away from the solidification interface under the concurrent actions of the Marangoni migration, Stokes motion of the droplets as well as the movement of the sample. Al-3.4%Bi alloy solidified under such conditions shows a size distribution of the minority phase particles with two peaks. The average radius of the minority phase particles depends on the solidification velocity V_0 exponentially according to. With the decrease of the solidification velocity, the dependence of the average radius of the minority phase particles varies towards to .