Recently, a new bulk amorphous system Cu40Ti30Ni15Zr10Sn5 has been reported. The glass transition temperature Tg is 735 K and the crystallization temperatures are 780 K (Tx1) and 816 K (Tx2), respectively. The phase transition of the Cu40Ti30Ni15Zr10Sn5 amorphous alloy annealed at 735 (Tg), 758 (Tg+Tx1)/2, 780 (Tx1), 900 and 1000 K, respectively, was studied by X-ray diffraction (XRD), differential scanning calorimetry (DSC) and high-resolution analytical electron microscopy (HRATEM). The crystallization of the alloy proceeds by the process Am (amorphous state) →CuTi+Cu10Zr7→Cu3Ti+Cu2Ti+CuTi2 +Cu10Zr7. Two exothermic peaks are observed in the DSC curve of the as-quenched sample, corresponding to the reactions of Am→CuTi+Cu10Zr7 and CuTi+Cu10Zr7→Cu3Ti+CuTi2+Cu2 Ti+Cu10Zr7, respectively. During the first-stage of crystallization, a significant redistribution of Zr and Sn was recognized by a composition analysis with energy dispersive X-ray spectroscopy (EDS), implying that the phase transition is controlled mainly by the rearrangement of the solute elements Zr and Sn. The necessity of this long-range rearrangement of Zr and Sn in the first-stage of crystallization seems to be one of the reasons for the high stability of the supercooled liquid in the present alloy system.