The deformation behaviours of A356 alloy with various initial grain structures, from a very coarse dendrite to a fine equiaxed structure, prepared at different pouring temperatures were investigated. Cylindrical specimens were compressed to a height reduction of 6 mm at an initial strain rate of 1.39 × 10-3/s and then jumped to 1.39 × l0-2/s. The instantaneous strain rate sensitivity was calculated under iso-structure conditions. Three very different deformation behaviours have been observed, which are believed to have resulted from their different starting microstructures. The materials produced at low pouring temperatures evolve into a fine globular structure after isothermal holding at semisolid region and shows a very low compression stress, while the materials cast at high pouring temperatures remain as coarse and dendritic in its microstructure and have one order of magnitude higher compression stress. The macrostructural breakage and microstructural segregation of hot compressed samples were analyzed and the deformation mechanisms were discussed.