Description: LeoCrystal is a software program that performs numerical Monte Carlo modeling of reaction on the surface of the growing crystal for illustrative educational and research purposes. With the help of this program you can estimate influence of effective geometrical sizes of structure elements of the crystal and thermodynamic parameters of the process on the topology and kinetic of the crystallization. As soon a process of crystal growth is presented itself in practically all major technology processes an in depth understanding of complexity of it is essential for professional research in different area of applied science. Particular interest it could be for the nanotechnology investigations permitting off site optimizing design on the molecule scale level. The creative presentation of the surface includes a three-dimensional perspective projection and stereo view. It makes this software a great illustrative toll for teaching of the concept of phase transition for the students of very broad ranges of initial background. Very moderate scientific background is needed to understand a material in this section. Preliminary runs of the program will make a great help to easy accommodate basic concepts. The crystal growth is a phase transition process with sharp border between it and initial feeding phase like a liquid, gas or plasma. The structure element (molecule) of the crystal could be determined as a minimum part of it when a reaction of incorporating its in the crystal will effect with changing energy of the whole system that will be equivalent to the condensation energy of the corresponding mass of crystal. In simple words the molecule is the minimum part of the crystal that behaves as a whole crystal. The principal difference of behavior of molecules in liquids is based on the principle of the long order in the crystals structure. Each molecule has exact position relatively to the other in crystal.