The diploma thesis focused on population dynamics of small terrestrial mammals in the Mláčik NNR, which is situated in the Kremnické vrchy Mts. The thesis is focused on two dominant small mammals species - yellow-necked mouse (Apodemus flavicollis) and bank vole (Clethrionomys glareolus) in nature fir-beech forest. Basic data for research were collected during tree trapping seasons in years 2017, 2018 and 2019. Data were obtained by using of method CMR, where small terrestrial mammals were trapped to Chmela-type live traps. Based on data about trapped individuals, some population characteristics of both species were calculated, such as abundance and density for each year of survey. In area of the Mláčik NNR the research of small terrestrial mammals has been realising continuously since 2006, so it was possible to make analysis, comparison and evaluation of population density values for both species in period 2006 to 2019. Based on this data is possible to analyse population dynamics and to review inner and outer factors, which can have some influence on density process access of both studied small mammals species. Here were together recorded 90 trappings of Apodemus flavicollis and 38 trappings of Clethrionomys glareolus during the research period. There are quite big yearly differences between trapped spieces and it is possible to see direct relationship between distribution of the two dominant rodent species and the high beechnut supply in the given period.