Magnetic domain wall dynamics in garnet film was investigated in a wide range of drive fields (H
z) using high speed photography. It is shown that the wall widening, the so-called dynamic diffuse wall begins at

where the velocity, after reaching its saturation, starts to increase again. The dynamic domain wall formation can be interpreted on the basis of the theory of spinodal decomposition, i.e. as the quasi-periodic fluctuation of the wall surface.