The slope failure so far has been occurred due to the heavy rainfall, and induced losses of a human life and properties. In order to prevent these damages, the measurement and the evaluation for the strength characteristics of the ground such as the shear strength and the angle of internal friction should be required. The direct shear test (DST) which is one of testing methods for this has been generally used, and has several experimental limitations. In particular, it should be recognized that there is no the standard of the opening size between the shear boxes in the DST, and the strength characteristics can vary according to the opening size. Thus, Kim et al. (2012) suggested that the opening size is determined based on the Threshold-Line. In this study, the Threshold-Line (TL) proposed by Kim et al. (2012) was examined through the distinct element method (DEM) simulation. As a result, the shear behaviors in the DEM simulation are similar to the experimental results by Kim et al. (2012). Through the results of the DEM simulation, it can be confirmed that the TL can be a standard for the opening size in the DST.