The method of a determination of the Primary Cosmic Ray mass composition is presented .
Data processing is based on the theoretical model representing the integral muon multiplicity spectrum as the superposition of the spectra corresponding to different kinds of primary nuclei .
The method consists of two stages .
At the first stage , the permissible intervals of primary nuclei fractions f _ { i } are determined on the base of the EAS spectrum vs the total number of muons ( E _ { \mu } \geq 235 GeV ) .
At the second stage , the permissible intervals of f _ { i } are narrowed by fitting procedure .
We use the experimental data on high multiplicity muon events ( n _ { \mu } \geq 114 ) collected at the Baksan underground scintillation telescope .
Within the framework of three components ( protons , helium and heavy nuclei ) , the mass composition in the region 10 ^ { 15 } -10 ^ { 16 } eV has been defined : f _ { p } = 0.235 \pm 0.02 , f _ { He } = 0.290 \pm 0.02 , f _ { H } = 0.475 \pm 0.03 .