The undercooled solidification of (Co₆₀Sn₄₀)_100-xNb_x(x=0, 0.4, 0.6, 0.8, at%) single phase alloys was performed to investigate the changes of Co₃Sn₂ phase growth morphology.Results show that the Co₃Sn₂ phase grows with fractal seaweed morphology at small undercooling (x=0, 0.4), and transits to dendrite as the content of added Nb increases to 0.6at%, and then returns to fractal seaweed (x=0.8) as a response to the changes in interfacial energy anisotropy and kinetic anisotropy. With the increase in undercooling, the growth morphology of Co₃Sn₂ phase in (Co₆₀Sn₄₀)_99.4Nb_0.6 alloys returns from dendrite back to factual seaweed at undercooling larger than 28 K and then transits to compact seaweed at undercooling more than 143 K. The minor Nb addition slightly increases the growth velocity of Co₃Sn₂ phase at low and intermediate undercooling but obviously decreases the growth velocity at large undercooling. The sharp increase in the growth velocity is corresponding to the transition of Co₃Sn₂ phase growth morphology from fractal seaweed to compact seaweed.