Surfaces of L-Threonine (L-Thr) seed crystals placed in a supersaturated DL-Thr solution were observed under ail optical microscope and an AFM to understand tire effect or washing seed crystals with water on the purity drop during preferential crystallization. Similarly to a previous study, thin plate-like crystals of D-Thr were found to appear by the surface nucleation mechanism on the {210} faces or the seed crystals, when they hall not been washed. For the case of washing, the {210} faces disappeared, and the {110} and {100} faces appeared in place of the {210} faces, and they grew to recover to form {210) faces. During the recovery, thin D-Thr crystals did not appear, but after tire complete recovery were round to exist oil the surface. From these observations. surface nucleation as the retardation mechanism of purity drop by the washing of seed crystals was proposed.