In current practice, vertical ground motions are often ignored in design of concrete gravity dams as they are considered not to effect on the performance of the dams. In this study, a seismic assessment is completed by modelling a typical concrete gravity dam located in a seismically active region using finite element analysis software PLAXIS 3D. This was done to see if varying the seismic parameters affects the overall performance of the dam, and whether the vertical peak ground accelerations influence the design process. The model dam is exposed to a variation in seismic parameters to analyze its response using PLAXIS 3D’s pseudo static analysis function. The dam’s performance against two failures was assessed: global factor of safety and maximum vertical deformation. This analysis is based on six seismic load combinations (100 % horizontal with 0 % vertical, 100 % horizontal with 30 % vertical, 100 % horizontal with 67 % vertical, and vice versa of these combinations) of horizontal and vertical peak ground accelerations. Results show that although variation in the vertical component does not have a critical effect on the global factor of safety of the dam, it affects the vertical deformation under the dam. When 100% vertical PGAs are applied, they result in 56mm deformation compared to 49mm when they are ignored, 52mm when reduced to 30%, and 54mm when reduced to 67%. Varying the horizontal PGAs was seen to have negligible effect on the vertical deformations, however horizontal PGAs were found to be critical in design for the global factor of safety. When 100% horizontal PGAs were applied, the factor of safety dropped to its lowest value of approximately 2.0 in all three cases despite an increase in vertical PGAs. This illustrates that the vertical component of an earthquake affects the design against settlements under the dam, if not the slope stability failures, and hence cannot be ignored in the overall seismic design process.