We report the systematic characterization of metal nanocrystal formation on ultra-thin tunnel gate oxide (2∼3nm) for memory applications. To get a high density and small average size of nanocrystals, the process parameters including annealing temperature, initial film thickness, and substrate doping are investigated for Au, Ag, and Pt nanocrystal formation with Si nanocrystal structure as control samples. The observation of nanocrystal formation by scanning electron microscope (SEM) shows that annealing below melting temperature of deposited film contributes to the reshaping of nanocrystals, while the initial film thickness to actual nanocrystal growth. In addition, the Schottky charge effect from substrate doping is not negligible if the tunnel oxide is thin. Controlling the process parameters, Au, Ag, and Pt nanocrystals of 4.0×1011cm-2, 2.8×1011cm-2, and 2.4×1011cm-2 can be formed with mean size of 6.2nm, 6.6nm, and 8.0nm, respectively. The observation of nanocrystals by scanning transmission electron microscope (STEM) shows that nanocrystals are spherical and crystalline. Metal contamination to the Si/SiO2 interface is also closely monitored with many process recipes of metal nanocrystal formation on 2∼3nm oxide showing atomically clean interface. Electrical evaluation of nanocrystal formation is carried out by C-V measurements of metal-oxide-semiconductor (MOS) capacitors with embedded metal nanocrystals.