This study presents the athermalization of a forward looking infrared system which is one of the task of a optomechanical engineer. Three-dimensional modeling and optical design of a forward looking infrared system are done. Thermal and structural analyses are performed by using the finite element method. Initial conditions and obtained results are verified by a laboratory study. The system parameters are optimized for ensuring the system to perform at different environmental temperatures by determining temperature distributions, expansions and contractions.