Controlling of a nucleation event is one of the important factors for preparing nanocrystalline materials via undercooled melt. We investigated nucleation behavior of an undercooled. Pd40Cu30Ni10P20 melt, which exhibits exceptional high glass-forming ability and thermal stability using an in-situ high-resolution electron microscopy. As a result, generation and annihilation of a primary Pd15P2 phase with a diameter of about 15 nm was observed in the undercooled particle sample of about 25 nm, Not only the origin for high glass-forming ability of the alloy but also the control method of a nucleation in order to prepare a nano-crystalline material via undercooled melt are discussed.