Kallat tunnel is located at the end of Mashhad-Kallat motorway and constructed in (1989) at the northeast of Kalat synclinal. During the tunnel excavation, some parts of tunnel was collapsed. Kallat tunnel was excavated in sandstone, limestone and marl. This paper presents the investigation of structural characteristics and stability assessments of the Kallat tunnel for widening it. Therefore, the rock mass rating (RMR), geological strength index (GSI) and rock mass quality (Q) systems was used for estimation of quality and quantity of rock masses properties around the tunnel. In order to estimate strength parameters of rock mass (compressive strength, tensile strength, deformation modulus and Hoek-Brown constants), Hoek-Brown failure criterion was used. Also finite element method (FEM) was used for stability analysis of Kallat tunnel. According to these data, the beginning section of the tunnel has the best stability and the end section of the tunnel has the worst stability conditions. The preliminary support design of this tunnel was analyzed by empirical method using the rock mass rating (RMR) and rock mass quality (Q) systems.