This paper presents results of a comparative numerical model study at a highway tunnel in Turkey. The tunnel was opened in a sandstone-siltstone rock mass having clear systematic discontinuity sets. In practice, effects of discontinuities of rock mass on tunnel stability are usually disregarded in finite element (FE) models, which are widely used for design of underground structures. To compensate this deficiency, rock masses' geotechnical properties are reduced to simulate effect of discontinuities in FE models. Main objective of this study is to show the effect of systematic and persistent discontinuity sets to FE numerical modeling when discontinuities exist. On this account, two different conditions were modeled to analyze stability conditions in a comparative manner. FE numerical analysis software "Phase(2) 8.0" was used for modeling. Results have shown that presence of discontinuities have a significant effect on tunnel stability and model results when FEM software is used for discontinuum media. As a conclusion, in case of using FE numerical models without taking the discontinuities into account would definitively lead to erroneous results in assessing tunnel stability. (C) 2014 Elsevier Ltd. All rights reserved.