The Cu-Ta system is a simple binary system with interesting properties, such as the formation of amorphous alloys and high-energy mechanical alloying capacity. Due to a high melting temperature of tantalum, experimental information about phase diagram is limited to one liquidus point. The ab initio and molecular dynamics approaches were used in this work for determination of thermodynamic properties of solid and liquid phases of Cu - Ta system. After that, the phase diagram was modeled with the aid of the Calphad method. As a consequence of these steps, a new thermodynamic description of the Cu - Ta system is proposed in this paper.