Dating of metamorphic processes is difficult due to genetic heterogeneity of metamorphic rock. Problem of dating is often connected with long and multistage proceeding of metamorphism. Time of process is determined by indirect method. This is not reliable approach. Time of metamorphism should directly be determined with the use of minerals, metamorphic origin of which is clear. Pb-Pb-investigation of metamorphogenic mineral, sillimanite have been performed. Results of the investigations have been presented. Method of step-by-step dissolution has been applied. Isotope investigation makes it possible to determine the time of peak of metamorphism.