Grain size evolution for γ-TiAl based alloys with aluminum content ranging from 43 to 52 at.% were cast at constant superheat and characterized. Experiments show that the sooner the peritectic phase transition occurs, the finer are the equiaxed grains. An accurate estimate of the number of surviving dendrite fragments and fragmentation rate is performed, which can be used to predict the position of the columnar-to-equiaxed transition in the cast ingots. For alloys solidifying with the β pro-peritectic primary phase, the fragmentation rate significantly increases when the onset of the peritectic reaction is earlier. A mechanism based on the peritectic reaction is proposed for the observed grain refinement and is discussed on the basis of the fluid flow, mushy zone permeability and the influence of the peritectic reaction.