The effect of hydrogen treatment (HT) on the solidification structures and mechanical properties of Ti-47Al alloys were studied using different contents of hydrogen. Experimental results indicate that together with the refinement of lamellar structure, the grain size of Ti-47Al alloys is reduced. With the hydrogen addition increasing to 1%, the grains of Ti-47Al alloys are refined from 1000 to 100 pm, and the average lamellar spacing of Ti-47Al alloys is decreased by approximately 50%. The hardness, compressive strength and yield stress of Ti-47Al alloys are improved due to the refinement effect of HT on grains and microstructures. The refinement effect of HT on cast TiAl alloys is found to be related to the enhancement of constitutional supercooling induced by hydrogen ahead of the solid liquid interface. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.