Rapid dendritic growth in undercooled liquid Ni80Cu10Co10 alloy has been investigated. This liquid ternary alloy was undercooled by a large degree up to 335 K(0.2TL) with electromagnetic levitation method. The rapidly solidified microstructure is revealed to be α-Ni solid solution phase by X-ray diffraction and differential scanning calorimetry. With the increase of undercooling, the microstructure is drastically refined. Its morphology transfers from coarse dendrite into equiaxial grains when undercooling exceeds 110 K. The solute microsegregation is suppressed with the enhancement of solute trapping effect under high undercooling condition. The dendritic growth velocity was measured as a function of undercooling. At extremely large undercoolings, the dendritic growth velocity of α-Ni phase is restrained by the solute diffusion of Cu and Co, and even decreases once undercooling exceeds 284 K.