A356 aluminum thin plates with a thickness of 1.2mm were produced via thixoforming process under different temperatures ranging from 585°C to 610°C. In addition to large primary α-Al grains, unique fine secondary α-Al grains (equivalent diameter below ~8μm) were revealed through microstructural examinations. These secondary α-Al particles exhibited irregular morphology at 590°C, which gradually transformed into equiaxed/globular grains at higher thixoforming temperatures of 605°C and 610°C. On the other hand, their size (the equivalent diameter of the grains) continuously increased from 3.2±0.16μm at 590°C to 8.1±0.4μm at 610°C. Energy dispersive X-ray spectroscopic analyses showed that the Si concentration gradually increased within these secondary α-Al grains. A quenched modified morphology consisting of angular and finely branched eutectic silicon was observed at the thixoforming temperature of 590°C, which seemed that it changed to a fibrous morphology at 605°C and 610°C. By increasing the thixoforming temperature, the elongation of the thixoformed plates monotonically increased to 12.6±0.4% at 610°C, whereas the ultimate tensile strength reached its highest value of 280±12MPa at 600°C then experienced the sharp reductions at 605°C and 610°C.