The modeled Environmental Control System is based on air cycle and is designed for aeronautical use. The aim of this thesis is to simulate the behavior of the ECS in different operational conditions with a model developed in Matlab/Simulink. The modeling work starts with a simple and partial model and step by step it was improved with other components and details. After every step of modeling there was a verification phase where the outputs were validated. To validate the Air Cycle Machine outputs and the heat exchanger outputs was dedicated particular attentions. The results of the final model were validated with empirical data from the test rig.