In this article, the effect of reservoir length on seismic performance of gravity dams to near- and far-fault ground motions is investigated. For this purpose, four finite element models of dam-reservoir-foundation interaction system are prepared by using the Lagrangian approach. In these models, the reservoir length varies from H to 4H (H: the height of dam). The Folsom gravity dam is selected as a numerical application. Two different ground motion records of 1989 Loma Prieta earthquake are used in the analyses. One of ground motions is recorded in near fault; the other is recorded in far fault. Also, the two records have the same peak ground acceleration. The study mainly consists of three parts to assess the effects of reservoir length on the seismic performance of the concrete gravity dam. In the first part, the linear time-history analyses of the four finite element models prepared for the Folsom gravity dam are performed. In the second part, the seismic performance of the dam is evaluated according to demand-capacity ratio and cumulative inelastic duration. Finally, the nonlinear time-history analyses of the finite element models of the dam are carried out by using Drucker-Prager yield criteria for dam concrete. It is seen from the analyses results that the seismic behavior of the concrete gravity dams is considerably affected from the length of the reservoir. The reservoir length of 3H is adequate for concrete gravity dams. The selection of ground motion is on of the important parts of seismic evaluation of gravity dams. Also, the frequency characteristics of the ground motion having the same peak ground acceleration affect the seismic performance of the dam. The near-fault ground motions are generally creates more stress on the dam body than far-fault ground motions. The used performance approach provides a systematic methodology for assessment of the seismic performance and necessity of nonlinear analyses for dam systems.