Recently, a new charge percolation mechanism (CPM) of the Ziegler-Natta (ZN) polymerization of olefins by supported transition metal (Mt) complexes has been presented: a macromolecular chain is formed by polymerization of the monomer cluster (nM) adsorbed on the support (S) between two immobilized Mt ions, some in the higher (Mtn+1, i.e. acceptors) and the other in the lower (Mtn-1, i.e. donors) oxidation state: (Mtn-1...nM...Mtn+1)/S → (Mtn Mtn)/S + polymer. A special computer program «Lattice» has been developed to simulate olefin polymerization based on CPM using a Monte Carlo procedure. The effects of reaction conditions (Mt concentration, Mt/S ratio, sequence of chemical components addition and time) of ethylene and propylene polymerization by various Mt precursors (TiCl4/MgCl2, CrOx) and supports (MgCl2, SiO2) on molecular mass and molecular mass distribution can be predicted by simulation and confirmed by published experimental results.