The microstructure and composition of the liquid along the mushy zone of superalloy Inconel 718 that was directionally solidified at various solidification rates after heated under different remelting temperature have been investigated by scanning electron microscope (SEM) and energy dispersion analysis spectrometry (EDAX) techniques.The content of Nb and Mo in the interdendritic liquid along the mushy zone was presented. The liquid-density difference and relative Rayleigh number in the interdendritic liquid were calculated. It was found that when the remelting temperature increased from 1600℃ to1700℃, the segregation of Mo became more serious, the liquid-density difference became higher at the beginning solidification and the maximum relative Rayleigh number (Rar) increased greatly which indicated the tendency of thermal solutal fluid flow became larger and mushy zoen was more unstable.