It is possible to know the information about living ethnic groups and changes in ethno-linguistic situation in a certain area not only from historical data but also from linguistic ones, primarily from onomastics. This article deals with the problem of origin of some geographical names: Myory, Postavy, Sharkovshchina. These towns are situated in the western part of Vitebsk Region of Belarus. Oikonyms Myory and Postavy have a hydronymic origin,the topographic basis for these geographical names were the names of the lakes ( or parts of them) near which these towns arose. In the name of Lake Myory, which gave the name to the town Myory, a Finnish background could be traced,for a period of several centuries a Finnish name “Maijaura” (maij/jaura:”maij”-a beaver and “jaura”-lake) has been transformed into Belarussian Myory. The name Postavy most probably came from a Lithuanian word “postovis”, which means ‘a deep place in the river where the water flows slowly’. In the Belarussian language this Lithuanian word has been transformed into Pastavy. Sharkovshchina has a patronymic origin from a Lithuanian antroponym “šarka”. Toponyms, which have been analyzed in this article, are evidence that in ancient times the territory of today’s Vitebsk Region was inhabited by Finnish tribes, after them came Baltic tribes and later Slavonic tribes.