Both glass fluxing and cyclic, superheating techniques were adopted to effectively undercool the Cu70-Ni10 alloy in vacuum. Within the undercooling range of 21 K to 270 K, the microstruoture evolution of the alloy was investigated. When the melt was undercooled to △T>△T3(210 K) , the grain refinement took place abruptly. Based on the observation of the solidified microstructure, the microchemical-analysis and the calculated results with HCT model, it is found that the secondary grain refinement mechanism consists of two stages. The dendrite is, firstly, broken into fragments owing to the stress caused by uneven shrinking during rapid solidification, then the fragments, under the driving foree of surface and strain energies, merge through the migration of boundaries, i. e. recrystallizalion, thus leading to the formation of secondary granular-crystalline. Both the glass fluxing and cyclic, superheating techniques were well undercool the Cu70-Ni10 alloy in vacuum. Within the undercooling range of 21 K to 270 K, the microstruoture evolution of the alloy was investigated. When the melt was undercooled to ΔT> Based on the observation of the solidified microstructure, the microchemical-analysis and the calculated results with HCT model, it is found that the secondary grain refinement mechanism consists of two stages. The delta T3 (210 K) dendrite is, first, broken into fragments due to the stress caused by uneven shrinking during rapid solidification, then the fragments, under the driving foree of surface and strain energies, merge through the migration of boundaries, ie recrystallizalion, thus leading to the formation of secondary granular-crystalline.