Crystal growth in the undercooled DD3 single crystal super-alloy is investigated in this paper. With the increase of undercooling, two kinds of dendrites are obtained. When DeltaT < 30 K, dendrite growth is dominantly controlled by solutal diffusion, and the as-solidified morphologies are similar to the conventional as-cast highly branched dendrites. In the undercooling range of 78-150 K, the severe solute trapping that originates from high dendrite growth velocity weakens the effect of solutal diffusion on the dendrite growth. Thermal diffusion, controlled by heat gradient, instead of solutal diffusion, is found to become the key factor in determining the formation of largely developed fine dendritic morphologies. Furthermore, the measured dendrite arm spacing are explained by employing the current dendrite growth theories.