The paper experimentally investigated the effects of grain refinement on the rheological response of At and hypoeutectic Al-Si alloys. Selected refiners included K2TiF6, K2TiF6 plus graphite and Al-5Ti-B. The apparent viscosity of semisolid Al alloys was measured during solidification. C Samples at different solid fractions were quenched to observe the microstructure. It was found that grain refinement drastically lowered the apparent viscosity of Al-Si alloys. Among selected refiners, the effect of Al-5Ti-B was the best. The effect of K2TiF6 plus graphite was better than that of K2TiF6. Silicon contents in Al alloys affected the apparent viscosity. With increasing silicon content the apparent viscosity decreased, resulted from promotion of silicon to both refining effects of titanium and boron. (c) 2007 Published by Elsevier B.V.