Abstract: A method for the simultaneous modernization of a plant for ammonia synthesis and a plant for urea synthesis, provides—inter alia—the arrangement of a carbamate synthesis section and a carbamate decomposition section, in order to obtain a predetermined amount of carbamate in aqueous solution and of hydrogen and nitrogen in gaseous phase which are fed to the existing sections for urea synthesis, respectively ammonia synthesis. Thanks to this method of modernization it is possible to remarkably increase the production capacity and at the same time to reduce the energy consumption of the urea and ammonia plants without being forced to replace or anyway overload the existing sections of decarbonation, methanation and compression, in general.