The headrace tunnel alignment of the Seti Khola Hydropower Project passes through the Lesser Himalayan geological region situated in Gandaki Province, Nepal. The 6 m wide and 3105 m long inverted-D shaped headrace tunnel traverses through the low-grade metamorphic rocks of the Kuncha Formation (phyllite, metasandstone, quartzite and their intercalations). The rock mass condition encountered during the excavation of the tunnel from the inlet side was poor to very poor quality where frequent shears and/or fault zones are met. Hence, the rock mass is highly deformed, schistose, sheared and thinly foliated. In this study, the stability condition of the headrace tunnel has been investigated based on lithological variations, topography, rock mass conditions and in situ stress conditions. In addition, the results from field as well as laboratory investigations have been used to define the rock mass parameters. The stability analysis of the tunnel and support design was carried out by using analytical as well as numerical approaches. The input parameters used for the analysis were sensitized and the stability condition of the tunnel is validated based on the data collected during investigations and tunnel excavation. The design of the support is continuously updated and the rock support is used as per ground conditions encountered during excavation. It is concluded that proper field, as well as laboratory investigations followed by stability analysis using numerical modelling, could provide a reliable design for tunnelling in the weak rock mass.