Direct numerical simulations of solid-layer crystallization from the binary melt of fatty acid were carried out to predict the crystal purity. The melt flowed between horizontal parallel plates, and the solid layer was formed over the cold wall. The transient profiles of flow velocity, temperature and solute concentration during solid-layer crystallization of melt were calculated numerically by using a non-orthogonal coordinate system. The solute concentration profile inside the crystal layer was predicted theoretically without using any empirical parameters on the basis of the new theory of the solute distribution at the solid-liquid interface, which was proposed recently by the present authors. It was found that the purification (or solute-distribution) kinetics could be classified into three different types: the complete purification, the intermediate purification and the poor purification. The interfacial solute distribution factor for the intermediate purification was inversely proportional to the one-half power of dimensionless growth rate defined by the ratio of crystal growth rate to the mass transfer rate at the solid-liquid interface, The dimensionless growth rate was proportional to the first power of the driving force of mass transfer for the complete purification, and to the second power of that for the intermediate purification. The crystal purity was found to be basically determined by heat and mass transfer rates at the solid-liquid interface. (C) 2002 Elsevier Science B.V. All rights reserved.