The crystallization behaviour of Ni51Zr49 amorphous alloy has been studied by the transmission electron microscopy (TEM). It has been found that a metastable phase, NiZr, in rose-like morphology with C-centered monoclinic lattice, a = 0.3268, b = 0.4101, c = 0.5224nm and β= 71.8° in amorphous matrix in the earlier stage of the crystallization, The following orientation relationships between the metastable phase and the stable phase, NiZr, exist: 100NiZr＇//100NiZr 001NiZr＇//ll0NiZr. NiZr＇ phase is regarded as a metastable state of NiZr phase and transforms to NiZr phase gradually with an increase in temperature. A small quantity of Ni10Zr7 phase formed in the final stage of the crystallization. Both NiZr and NiZr＇ have complex twins with two principle typies, one of which is 180°twins with the 02l direction of NiZr or NiZr＇ as axis and another 36°rotation twins with 001 of NiZr or 010 of NiZr＇ as axis. The total amount of twins decrease gradually with the increase in temperature.