Since their discovery by Lijima in 1991[1], carbon nanotubes (CNTs), are considered a new generation of reinforcement [2]. Their "nano" size structure makes them potentially free of defects, which provides them with excellent physical properties [3,4]. There are two main nanotube types: single wall nanotubes (SWNT) and multi wall nanotubes (MWNT). These last ones consist in several concentric walls, one inside the other. In a composite, one the most important factors that condition their mechanical performance is the interfacial tension between matrix and reinforcement. In general, the loads in a composite structure are introduced to the matrix and then are transferred to the reinforcement through the interface [5]. Therefore, the interface can be defined as the region, surrounding the reinforcement, where this stress transfer takes place. The properties of the composite depend on the properties of this region and its ability to transfer the load efficiently. This work proposes a new formulation to predict the mechanical properties and mechanical behaviour of nanotube-reinforced composites. The formulation is based on the mixing theory [6]. It obtains the behaviour of the composite from the mechanical performance of its constitutive materials: matrix, carbon-nanotube and the interface that bonds both of them.