This thesis work started during an internship at Leonardo Aircraft Division Spa. The study was focused on the application of processes, methodologies and tools of System Engineering in the life cycle of an Environmental Control System (ECS) for UAV aircrafts. The main task is to show the process of development of the system through: Functional Analysis, Performance Analysis, Cost Estimate. The Functional Analysis has been made by means of the IBM Ration Rhapsody tool that allows a functional analysis using the SysML language, which has been implemented following a Harmony methodology. The Performance Analysis has been made by means of Siemens AmeSim, which allows a simulation of the two architectures, in order to size the system in both the two architectures analyzed that are: Vapor Cycle and Air Cycle.