In order to understand the effect of initial supersaturation on the mechanism of purity drop during preferential crystallization, two sets of experiments were carried out with a threonine-water system. One set was the observation of the [210) faces of L-Thr seed crystals placed in solutions at various supersaturation levels of D-Thr, and the other was preferential crystallization on the basis of the observation results. From the observation, needle-like D-Thr crystals, which nucleated in the bulk solution, were observed in supersaturated DL-Thr solutions at supersaturation of D-Thr (sigma(D)) below 0.19, while thin plate-like D-Thr crystals were found to appear on the {210} faces when sigma(D) was above 0.21. During preferential crystallization for the solution of initial supersaturation of sigma(i) = 0.16, needle-like crystals nucleated in the bulk solution were observed and the bulk nucleation of D-Thr was the mechanism of the purity drop. On the contrary, when the initial supersaturation sigma(i) was 0.21, thin plate-like D-Thr crystals were found to appear by surface nucleation. The surface nucleation of D-Thr was the mechanism of the purity drop. On the basis of observations and preferential crystallization, the mechanisms of the purity drop were concluded to be dependent on the sigma(i) in solutions.