Present study investigates the role of ultrasonic treatment (UT) on microstructure modification, porosity mitigation and their influence on mechanical properties of Sr modified A356 Al alloy. Addition of Sr in trace amount (500 ppm) to A356 alloy reduces the length of eutectic Si from 15.6 µm to 0.65 µm with a concomitant increase in porosity from 0.84% to 3.59%. Ultrasonic treatment of Sr modified A356 Al alloy retains the modification of eutectic Si (0.57 µm) and reduces the specific porosity volume to 0.86%. The effect of microstructure modification and porosity mitigation reflects on the mechanical properties of A356 Al alloy. While the Sr addition to A356 alloy reduces the tensile properties, UT of the Sr added alloy results in improved tensile properties. The mechanism of microstructure modification, porosity formation and UT assisted porosity mitigation are discussed in detail. Ultrasonic cavitation assisted degasification and bi-film breakage is proposed as the governing mechanism for porosity alleviation. © 2018 Elsevier B.V.