The effects of die temperature and plunger velocity on the soundness of semisolid cast products were investigated in regards to the flow behavior of the slurry of A356 alloy in the H-NCM rheo-diecasting process. The sound cast was obtained with the die-temperatures of above 220 . The sudden changes of casting velocity resulted in irregular microstructures with micro-porosity and shrinkage defects due to turbulent filling pattern. The high casting velocity at the end of filling has no effect on the castability and the formation of casting defects in H-NCM(Hong-NanoCasting method). The Sr modification provides much finer and spheriodized eutectic silicon particles.