Conventional laboratory tests provide macroscopic insights into rock failure, but the failure process is well-proven to be progressive and governed by the internal microstructure of rock. This revelation necessitates using advanced numerical techniques capable of investigating these internal features. Continuum grain-based modeling (CGBM) is a pseudo-discontinuum numerical technique that simulates the macroscopic response of rock from its microscopic behavior. In CGBM, the rock volume is synthetically represented as an assemblage of polygonal blocks. The behavior of synthetic CGBMs largely depends on assigned boundary conditions (BCs), an aspect requiring further investigation. To this end, this study utilizes the CGBM of intact Wombeyan marble, prepared by Li and Bahrani (2021), to study the effect of BCs. Specifically, four types of BCs were applied at the bottom end of the specimen, undergoing uniaxial loading: (1) BC-1: rollers with pin support in the middle, (2) BC-2: pin support, (3) BC-3: stiff platens at the bottom end, and (4) BC-4: stiff platens at the top and bottom ends. Following this, the behavior of the rock was analyzed during uniaxial loading. The results showed that the BCs employed in the models significantly affect the lateral stress distribution across the sample length. Specifically, BC-1 maintains uniform lateral stress, while BC-2 and BC-3 confine the lateral edge of the specimens. Likewise, BC-4 restricts lateral movement at both ends, resulting in increased lateral stress and unconfined compressive strength (UCS) of the specimens with such boundary conditions.