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Geosynthetics have been extensively used to reinforce soil structures, such as embankments, slopes, walls, foundations and roads. Proper evaluation of the interaction between geosynthetic reinforcement and backfill is important to understand the mechanisms of geosynthetic-reinforced soil (GRS) structures. Pullout tests have proven to be an effective way to study such interaction. In a pullout test, a geosynthetic reinforcement layer is buried in backfill within a test box. Vertical pressure is applied on top of the backfill to simulate the normal stress on top of the geosynthetic reinforcement in a GRS structure. The geosynthetic reinforcement is then pulled out from the backfill through an opening in the front wall of the box. The pullout test results are influenced by boundary conditions due to the thickness of the backfill, as well as the roughness of the interface between the backfill and the walls of the pullout box. This paper discusses the results of a numerical study performed to investigate the boundary effect on pullout test results. A two-dimensional numerical simulation was conducted using a finite differential method program, FLAC, using the Mohr-Coulomb model to describe the behavior of the backfill. The geosynthetic reinforcement was modeled as a linearly elastic and perfectly plastic material. The numerical model was calibrated and verified against pullout tests of geogrids. Boundary conditions, such as backfill thickness, and the roughness between the bottom of the backfill and the wall of the pullout box, and how these affect pullout test results are analyzed and discussed. The numerical results show that the pullout forces at the large pullout displacement calculated from the numerical simulation with the fixed bottom were closer to the measured pullout forces than those with the free bottom.