[en] For the disposal of high-level radioactive waste, it is required to develop a disposal concept. The geological disposal research group in KAERI had developed a reference disposal concept and now is studying for developing a Korean disposal concept. In order to develop a Korean disposal concept, the validation of disposal concept and performance safety analysis are essential. For the validation of disposal concept, it is necessary to construct an underground research laboratory. For doing that, the research team for the validation of disposal concept studies for constructing an underground research facility in KAERI. In order to design the underground research facility with computer simulation, disposal concept, rock characteristics, and topology should be considered. In this study, the reference disposal concept, which is necessary to be considered in the design of underground research tunnel, will be introduced first. After then, the important factors related to the underground research tunnel will be discussed. In the case of the site, where the underground tunnel are expected to be located, the surface topology is varying with thick weathered zone. In order to make the research modules in deep location with limited tunnel excavation, it is recommended to excavate a declined access tunnel. This study is for investigating the influence of different geological conditions, tunnel slope, tunnel size, and sequential excavation. In this study, mechanical stability analysis for the conceptual design of the underground research tunnel for the validation of Korean disposal concept had been carried out. Investigation of the influence of important parameters on mechanical stability was performed. The results from this study will be utilized for the geological investigation at the site, where an underground research tunnel will be located, as well as the design of the tunnel. Important conclusions from the study are as followings: (1) If the underground research tunnel is located at KAERI site, it is not expected to develop any plastic zone around the tunnel, mainly due to the shallow depth. The maximum stress will be compressive stress and might be as high as 5MPa. The exact location, where the maximum compressive stress will be developed, will be about 20m to the dead end of the tunnel. There is no difference on stress and displacement distributions between the case with simultaneous excavation and the case with sequential excavation. (2) It is expected to have stress release in the roof and floor after the excavation of the tunnel. Tensile stress would be developed at some areas in the floor. (3) There is no influence of weathered zone size on the stress and displacement distributions around the tunnel. It is, however, recommended to carry out further analysis with actually measured properties of the weathered zone for more accurate decision. (4) With increase of tunnel size, the stress around the tunnel was increased. In the case of von-Mises stress, the magnitude increase due to 1m increase of tunnel size was found to be only about 4%