Gadook tunnel with a length of 2887m is located in the railway track of Firoozkooh to Ghaemshahr, north of Iran. The length of this section was considered 12m in order to get initial stress around the main tunnel. The optimization of the cross-cut section was investigated for diameters 2.5, 3, 3.5 and 4m considering a good type of rock material, the pressure drop for each one was calculated and then 3m diameter cross-cut was chosen based on the calculated pressure drop. In order to analyze the stability, first, the geometry of the main tunnel and the cross-cut with the length of 12m and a diameter of 3m in Rhino software was constructed, then, the stability analysis was performed using FLAC3D software. The intersection of the main tunnel and cross-cut was designed trumpet like to prevent the additional pressure drop. The numerical calculations obtained by FLAC3D software revealed the displacement in the intersection and inside cross-cut is as small as 2 mm, so the cross-cut can be considered as self-stable. Since the cross-cut is designed for ventilation needs, in order to avoid pressure drop inside the cross-cut, the walls should be flattened. For this reason, a support system was proposed consist of 15cm lining for the cross-cut and 20 cm lining for the 25m inside the main tunnel around the intersection. To verify the support system is only for the need of ventilation and pressure drop in the cross-cut, the shotcrete support system with a 5cm thickness inside the cross-cut was investigated and minimal displacements were obtained again. Regarding a good rock material and stability inside the cross-cut, the step progress of 2m determined in order to excavate the cross-cut in six stages.