In situ observation on the interface instability and the solute transport boundary layer in front of the solid/liquid interface is carried out during the di rectional solidification of SCN-1.5at%Salol. Based on this, and in combination with the experimental results of SCN-1.1 wt%Eth, SCN-0.43wt%C152 alloys, the morp hological instability of a solidifying planar interface is systematically invest igated during directional solidification with and without the convection. It is found that the interface stability analysis in our previous paper describes corr ectly the time-dependent evolution of interface instability. Both the incubation time ti for instability and the incubation time tb for breakdown of the planar interface can be determined from the time-dependent instability so lution, and the convection obviously shortens the incubation time ti for i nstability and the incubation time tb for breakdown of the planar interface. The steady-state instability solution overestimates the unit amplitude devel oping rate by an order of magnitude, as compared to the experimental value, while the time-dependent solution underestimates the experimental result.