Abstract: Main applications of isotopic Silicon in new areas were briefly introduced. The work done by the author’s institute on Silicon isotope separation and measurement of ²⁸Si epitaxial wafer and diodes in early year was presented. For precisely measurement of Avogadro constant as basic metrology unit of kilogram using ²⁸Si, relation of isotope abundance and lattice constant deviation was reduced. Examples were calculated using the relation, thus the requirement of Silicon abundance was estimated.