The morphological transition from the A-type to D-type graphite (undercooled graphite) in cast iron has been studied using Fe-3.5%C-2.0%Si-0/0.1%Ti samples. The samples were prepared using a high frequency induction furnace flowing Ar atmosphere using 0.25% steel rods with or without Ti addition. The samples had Ti contents that ranged from 0 to 0.10% at 5 different levels by the addition of sponge titanium. The cooling curves of these melts were measured in a shell mold with an inside diameter of 30mm and 50mm height and in four BN-coated steel cup molds with a volume of 30ml each. The cooling curves were measured by CA thermocouples located at the center. The cooling curves were differentiated to determine the transition points, namely the onset and end points of the eutectic solidification. Three out of the four samples, solidifying in steel molds, were quenched during the eutectic solidification and their macro-structures and micro-structures were observed for the determining the solidification mode. The volume fractions of the D-type graphite area in the samples were measured using 30 microscope images of 50× magnification, and their eutectic temperature was also determined using their cooling curves. The volume fraction of the shell mold samples increased with the Ti addition from 5% to 55%, and if the Ti content was greater than 0.05%, the acceleration occurred with their maximum undercooling, ΔTMAX. The critical undercooling temperature, TA/D, and the critical solidification rate, RA/D, of the A-type to D-type graphite transition were determined by comparing the volume fractions to the solidification time. The ΔTMAX and TA/D values increased with the Ti addition. This is the main reason why the Ti addition accelerates the D-type graphite increase.