Seismic assessment of existing engineering structures in earthquake zones is very important. This paper presents seismic assessments of a highway bridge and an arch dam using non-destructive tests under ambient vibrations. Firstly, finite element models of these structures are developed and analytical dynamic characteristics are attained. Secondly, experimental measurements under ambient vibrations are performed and dynamic characteristics are extracted using Enhanced Frequency Domain Decomposition and Stochastic Subspace Identification methods. Experimental and analytical dynamic characteristics are compared and finite element models are updated to eliminate the differences. Lastly, the seismic assessments of the selected structures are performed using the updated finite element models under earthquake ground motions.