Abstract: The solidification microstructure of superalloy IN718 and the contents of Nb and Mo in liquid during solidification were investigated at cooling rates of 1, 3 and 6 K·min^-1 by SEM and EDS. The liquid density difference and relative Rayleigh number at different temperatures and different cooling rates were calculated. The results show that the contents of Nb and Mo in liquid and the liquid density difference increase with decreasing temperature due to segregation. The slower the cooling is, the heavier the segregation of Nb and Mo is, and the more liquid density difference at the same temperature. The maximum values of relative Rayleigh number exist at 1 330℃ at all the three cooling rates, with the decrease in cooling rate the maximum values of relative Rayleigh number raise dramatically, and freckle tendency increases. Freckles could form in large industrial ingots at a cooling rate less than 1 K·min^-1