Solidification experiments were carried out for Al-6Si alloys with different La addition levels. The results demonstrate that the average sizes of the alpha-Al grains and the eutectic Si have a tendency of first decreasing and then almost keeping constant with the La addition. About (0.04-0.06) wt% La addition is enough to achieve a satisfactory grain refinement and modification. An analytical model based on the Wilson equation, the Miedema model and the thermodynamics was proposed to investigate the mechanisms of the alpha-Al grain refinement and the eutectic Si modification in the hypoeutectic Al-6Si alloys with the trace La addition. The La effect on the grain refinement of alpha-Al and the modification of the eutectic Si are mainly attributed to the increase in the efficiency of the heterogeneous nucleation substrates to nucleate alpha-Al and the amount of La atoms in the eutectic Si. The grain refinement and the modification results achieve the maximum when the La concentration is near its maximum solid solubility in the primary alpha-Al phase.