Changes in mortality behavior in 1950s-1970s pose a fluctuation of Czech population's mortality trend and still have an impact on current level of life expectancy. The thesis focuses on mortality process in that period from cross-sectional and longitudinal approach and tries to identify how the mortality changes affected age intervals older than 30 and influenced cohorts which were in these age intervals in observed period. In the thesis Arriaga's decomposition method was used for quantification of mortality changes in particular age intervals and their affect on changes of life expectancy. The obtained results show that mostly age groups older than 60 were more sensitive to mortality fluctuations.