The effect of reheating to the semisolid state (soaking treatment) on the microstructure evolution of the A356 aluminum alloy prepared by ultrasonic melt treatment was studied in this paper. The results showed that in general the longer the soaking process the larger and the more round the grains obtained. Higher roundness occurs at shorter soaking times in the fine-grained ascast samples, and at longer times in the inhomogeneous or the coarser-grained as-cast structures. The optimum thixotropic condition (high roundness, 0.72, and small globule sizes < 90 μm) are achieved after 5 min. soaking in the samples treated by UST at 623 and 620oC, which is the typical soaking time dictated by the industrial practice in SSM. The amount of entrapped eutectic as observed after soaking treatments is uniquely very small, suggesting that the UST-treated ingots will have better formability in the semisolid state. The growth rate constants are substantially low: in the order of 479-748 μm3/s. These growth rate constants are much lower than those reported for MHDcast A356 ingots. The growth rates of the samples produced by UST in the liquid state (i.e., 626, 623 and 620oC. Note that liquidus temperature is 619oC) are lower than those of the samples treated in the semi-solid temperatures, i.e., 617 and 614oC. The Ostwald ripening is most likely the dominant growth mechanism in the UST-treated samples during the soaking treatments. These results reveal the feasibility and competence of UST as a potential route for thixotropic feedstock production.