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Phase separation and crystallization behavior in melt-spun Al63Si25Ni12 amorphous alloy have been investigated in the present study. Although there is no atomic pair with positive enthalpy of mixing, nm scale phase separation occurs in the as-melt-spun amorphous alloy, exhibiting double-halo diffraction pattern. The amorphous matrix consisting of two different nm-scale Al-rich and Si-rich amorphous phases polymorphically crystallizes into a metastable hexagonal phase by an interface-controlled nucleation and growth mechanism in the first step of crystallization. The metastable hexagonal phase transforms into the mixture of α-Al, Si and Al3Ni phases possibly by a ternary eutectoid reaction in the second step of crystallization.