In order to understand the effect of face crystallographic orientation of seed crystals on purity drop during optical resolution by preferential crystallization, experiments were conducted using two seed crystals having different faces and at different initial supersaturations. The threonine (Thr)-water system was used and one of the seed crystals used was rod-like surrounded by {210} faces while the other was plate-like with {100} faces. Except for the higher supersaturations needle-like D-Thr crystals were found to appear in the bulk solutions irrespective of the seed crystals, whereas at higher supersaturations thin D-Thr crystals were found only on the {210} faces of the rod-like seed crystals. These thin crystals were not observed on the {100} faces of the plate-like seed crystals at the same higher supersaturations, However with these seed crystals, nucleation of needle-like D-Thr crystals was observed, therefore the mechanism of purity drop depends on the face crystallographic orientation of seed crystals as well as the solution supersaturations. Based on the observed facts, surface nucleation of D-Thr is the cause of purity drop for the rod-like seed crystals at high supersaturations, while bulk nucleation predominates for other cases. The time at which purity drop began was also found to vary with these operating conditions showing the differences in the mechanism of purity drop.