•Porosity in the HPDC is inevitable and dominants mechanical property variability.•A critical pore size of 0.3 mm is acceptable for applications of HPDC castings.•The elongation of the A356 alloys increases as the largest pore size decreases.•The total volume porosity of the alloy declines with increasing elongation. To reveal the influence of porosity on the variability in mechanical properties of HPDC Al alloys, micro computed tomography was employed to investigate the morphology and 3D distribution of porosity in the tensile samples. Experimental results show that the variability in mechanical properties of HPDC AlSi7MgMn alloy is related to the pore size and total volume of the porosity. The maximum pore size is inversely proportional to the elongation of the alloy with T6 heat treatment, while the total volume porosity was found to decline with increasing elongation. A maximum pore size of approx. 1.3 mm in diameter was found to correspond to an elongation of 6.4%. Once its maximum size reduced to less than 0.3 mm, the elongation was found to improve to 9%-13.5% for the alloy. Compared to the average value of 8.8%, 236.6 MPa, 296.0 MPa for elongation, yield strength and ultimate tensile strength respectively for the porosity-free AlSi7MgMn samples produced by gravity casting, the HPDC AlSi7MgMn alloy has the similar strength level and improved elongation to an average level of 11.5%. This indicates that the porosity level is a determined factor to the mechanical property variability and its size less than 0.3 mm has no significant adverse effect on the mechanical properties of the alloy. The elongation improvement in HPDC AlSi7MgMn alloy is attributed to the finer grain size with an average value of 10 μm compared to the average value of 500 μm for the gravity casting AlSi7MgMn alloy, and to the reduced size and uniform distribution of porosity resulting from the subsequent refinement in grain size.