The buttress dams or abutment walls are slender structures in the line of dam site, so their ability to work under load, especially under seismic load in this direction is less effective than other directions. Therefore, the rigidity increasing of buttress dams in the line of damsite significantly improves their seismic resistance. This paper includes the analysis of the influence of rigidity-increased structures to the dynamic characteristics (natural-vibration period and frequency) of the concrete buttress dams.