Primary cellular/dendritic spacing of the solidified structures plays an important role on the microstructure control under rapid solidification conditions. In order to clarify the phase evolution and growth mechanism of peritectic reaction, here laser resolidification experiments, with different growth velocity, have been performed on Ti-Al peritectic alloys. By taking both the transverse and longitudinal sections of the directional growth cells/dendrites in the laser traces, the primary phase formation and average primary cellular/dendritic spacing, as well as the corresponding solidification velocity are quantitatively determined by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). With the solidification velocity increasing, the separating primary phase evolves from single-phase alpha, two-phase (alpha+gamma), gamma, and the corresponding primary cellular/dendritic spacing decreases gradually. All experimental observations of primary spacing evolution, in all single phase regions, agree well with that predicted by rapid dendritic growth model. (C) 2004 Elsevier B.V. All rights reserved.