Grain refinement may not only promote the formation of a fine quiaxed grain structure, which endows the Al alloy castings with good mechanical properties, but also cause a reduction in the casting defects, such as segregation and hot tearing, which has a dominating effect on the processability of Al alloys. It is, thus, essential for both the cast and wrought Al alloys. Although many techniques, e.g. mechanical vibration, electromagnetic stirring, ultrasound vibration, etc. may be used for the grain refinement nowadays, inoculation remains the most widely applied method in the industrial production due to its simplicity and high efficiency. For most Al alloys, Al-Ti-B master alloy is used as the grain refiner. Much work has been done to investigate the solidification behaviors of the Al alloys inoculated with Al-Ti-B master alloys since the 1970 s. Models were developed to describe the microstructure formation under the effect of inoculants. These researches clearly demonstrate that the grain refining efficiency or the heterogeneous nucleation rate is closely related to the concentration of solute Ti as well as the number density and size distribution of TiB₂ particles in the melt. One shortcoming of the previous research work in this field is that the kinetic behaviors of TiB₂ particles during the heating or cooling processes of the melt, i.e. dissolution/growth, coarsening and precipitation of TiB₂ particles, are neglected. Generally the size distribution of TiB₂ particles in the Al-Ti-B master alloy was used in the modeling and simulation of the solidification of Al alloys. In this work, solidification experiments were carried out to investigate the kinetic behaviors of TiB₂ particles in the melt and the effect of solute Ti. A model was developed to describe the kinetic behaviors of TiB₂ particles during the whole process from the beginning of the addition of TiB₂ particles to the melt until the solidification of the melt. Calculations were carried out according to the experiments conditions. The results demonstrate that TiB₂ particles may dissolve and coarsen during the holding temperature period, and grow during the cooling period of the melt. The kinetic behaviors of TiB₂ particles have an obvious effect on the grain refining efficiency of the master alloys. The addition of solute Ti can significantly suppress the growth/dissolution, the Ostwald ripening of TiB₂ particles and thus affects the grain refining efficiency of the master alloy.