The solidification microstructures and the mechanism of grain refinement of electrolytic low titanium Al alloys were investigated by means of the wedge-shaped sample, the directional solidification and the rapid solidification ribbon. The results show that the coarse columnar grains formed in pure Al are transformed into the equiaxed grains in electrolytic low titanium Al alloys. The grain refinement is resulted from the constitutional supercooling caused by Ti and heterogeneous nucleation of Al3Ti particles. Under the condition of normal cooling rate, the grains are refined by the increment of constitutional supercooling when the content of titanium is less than 0.2%. With the increment of content of titanium, the grains are mainly refined by heterogeneous nucleation of Al3Ti particles. The grain size is decreased with the increment of cooling rate. When the cooling rate is larger than 105 ℃/s, the grain size is decreased to 0.1-10 μm, the grain refinement is resulted from the larger cooling velocities mainly. After directional solidification, the equiaxed grains can be formed and the Ti element is distributed at the center of the grains.