Leg prosthesis in the market and in literature that actuate both knee and ankle joint are usually heavy and discarded by the patients for lighter semi-active ones. The purpose of this thesis is to study the possibility to realize a multi-joint under-actuated leg prosthesis based on Electro-Hydrostatic Actuation (EHA), as to have only one power unit for both joints, reducing the bulkiness and overall weight. Hydraulics motion transmission can serve an optimum role in such application, as power transfer between the components is an easier task as the motor does not have to be placed close to the actuated joint. The hydraulic circuit actuating the prosthesis is made up of a EHA unit composed of a fixed-displacement pump and an electric motor; two hydraulics actuators (double-acting hydraulic cylinders) are connected to the EHA and are in direct configuration (upper chamber connected to upper chamber).