The Al–2.5C master alloy is prepared to investigate the effect of the Al_4C_3 particle size distribution on the refining efficiency of the AZ31 alloy. The results indicate that the Al_4C_3 particles are potent nucleation substrates for primary a-Mg grains. With 1.0 wt% master alloy addition, the grain size is reduced from 204 to 70 lm. The grain refining efficiency of the Al_4C_3 particles on the AZ31 alloy is calculated to be 0.04%–0.75%. Such low refining efficiency is mainly attributed to the size distribution of the Al_4C_3 particles. The particle sizes are in the range from 0.18 to 7.08 lm, and their distribution is well fitted by a log-normal function. The optimum particle size range for significant grain refinement is proposed to be around 5.0–7.08 lm in the present conditions. The Al-2.5C master alloy is prepared to investigate the effect of the Al_4C_3 particle size distribution on the refining efficiency of the AZ31 alloy. The results indicate that the Al_4C_3 particles are potent nucleation substrates for the primary a-Mg grains. With 1.0 wt% Master alloy addition, the grain size is reduced from 204 to 70 lm. The grain refining efficiency of the Al_4C_3 particles on the AZ31 alloy is calculated to be 0.04% -0.75%. This low refining efficiency is mainly attributed to the size distribution of the Al_4C_3 particles. The particle sizes are in the range from 0.18 to 7.08 lm, and their distribution is well fitted by a log-normal function. The optimum particle size range for significant grain refinement is proposed to be around 5.0-7.08 lm in the present conditions.