The alloy with composition Ni-45.61Al-7.8Mo (at%) has been directionally solidified by liquid metal cooling (LMC) process. The effect of growth rate on the solidified microstructure such as interface morphology, dendrite growth and primary β phase precipitation, was experimentally investigated. The interface morphology of the β phase undergoes the evolution from plane front, cellular, thick dendrite to thin dendrite with increasing of growth rate for a given temperature gradient. The first dendrite spacing λ1 of the primary β phase increases gradually during the cellular growth, and decreases during the dendrite growth, while the second dendrite spacing λ2 decreases with increasing of the growth rate. The amount of primary β phase increases with the growth rate. And the analysis results show that the varieties of nucleation rate of NiAl caused by melt undercooling variation with different growth rates are responsible for the phenomenon.