Carbon nanotubes (CNTs), since their discovery by Lij ima in 1991 [1], 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), which are made of a single wall tube; and multiwall nanotubes (MWNT), which consist in several concentric walls, one inside the other. In a composite, one the most important factor is the interfacial tension between matrix and reinforcement. In general, the loads in a composite structure are introduced through 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 regíon and its ability to transfer the load efficiently. This work proposes a new formulation to predict the mechanical properties of nanotube-reinforced composites. The formulation is based on the mixing theory [6]. It obtains the properties of the composite from the mechanical performance of its constitutive materials: matrix, carbon-nanotube and the interface that bonds both of them.