Directional solidification studies have been carried out at the solidification rates of 0.8㎛/s-61㎛/s in the Ni₃Al composition. The microstructures formed at the solid/liquid interface were observed by the directional solidification technique that allowed quenching of the solidification interface. The dendritic interface was found at relatively high solidification rates and the planar eutectic interface at rates. Using this interface stability, the diffusion coefficient of liquid was estimated by the critical constitutional supercooling condition and it was in the order of 10^(-4)㎤/s which is about 10 times higher than usual. The equilibrium and non equilibrium eutectic structures were observed and the characteristics of these eutectic structures were also discussed.