High temperature flow behavior of Al-6.65Si-0.44Mg (all in wt%) (A356) alloy and A356+5wt% B4C composite were investigated by compression test at temperatures 470, 500, 530 and 570°C and strain rates 10-3, 10-2, 10-1 and 1s-1. Constitutive relationship was established by the prediction of materials constants α, β, n, Q and lnA. The variations in stable and instable regions of processing map were investigated in A356 alloy and A356+5wt% B4C composite. It was found that the existence of instability domain for A356 alloy, over the range of temperatures ~510-570°C and strain rates ~1- 4×10-2s-1, was extended to regime of temperatures ~470-570°C over the same strain rates upon 5% B4C reinforcement. The microstructural evolutions by stable and instable regions were studied in scanning electron microscope (SEM) and by the electron backscatter diffraction (EBSD) technique. © 2016 Elsevier B.V.