Abstarct: The depth of the subway directly affects the costs of tunnel excavation and the construction of metro stations. Minimizing these costs is a goal for all tunnel project managers. In this research, the effect of different depths has been investigated from both technical and cost perspectives. Then, the optimal depth for constructing subway tunnels has been determined according to both technical and cost points of view. From a technical standpoint, ground surface settlement is considered. Accordingly, if surface settlement is more than 25 mm, the overburden value should be increased until the settlement reaches less than 25 mm. Tunnel excavation costs and construction costs of metro stations also form a cost perspective in determining the optimal depth for the subway tunnel. For this purpose, by increasing the depth of subway tunnels and the distance of stations from each other, a comparison of the cost for constructing tunnels and stations has been considered. From a technical point of view, the results illustrate that the settlement exceeds the limitation in depths where the tunnel overburdens is less than the diameter. Therefore, tunnel excavation at such depths is not suggested. On the other hand, baxsed on the results of increasing the depth of the tunnel, the tunnel excavation costs initially decreased. After that, when the tunnel overburdens are more than twice the tunnel's diameter, it increases again. Also, with the increase of tunnel overburden, the construction costs of metro stations have increased. According to this study, the construction costs of a one-story station is equal to 195 billion IRR. Meanwhile, the construction costs of a four-story station are equal to 514 billion IRR. Regarding the total construction costs of tunnels and stations, the investigation on the outcomes of studying 17 km of the second line of Tabriz subway, which is done by mechanized tunneling method, clarified that a longer distance between the stations reduces the effect of depth on the total construction costs. Therefore, tunneling under the urban area where the distance between stations is usually from 800 m to 1200 m, the optimal depth for constructing tunnels and stations will be achieved when the overburden is more than the tunnel's diameter but less than one and a half times of it.