The mechanical properties of a cast product, and therefore its application, depend strongly on its inner microstructure. During the solidification step a change from columnar to equiaxed grain structure can occur. It is thus critical to understand the physical mechanisms of this transition in order to accurately predict and control its occurrence and the final grain structure morphology. This article reports on observations of the CET (Columnar to Equiaxed Transition) induced by a sudden increase of the pulling velocity during the directional solidification on a refined Al-3.5wt%Ni alloy by using synchrotron X-ray radiography. The influence of the pulling velocity on the blocking of the columnar structure is described. Next, the distribution of surface area and longitudinal asymmetry of the grains after CET are quantitatively characterized and discussed.