GaAs FETs are widely used for high-speed and low-power devices, especially in optical communication systems. To improve product reliability, it is very important to control mechanical stress in device structures. This is because the stress field changes the dopant distribution and shifts electronic characteristic of devices due to piezoelectric effect. There are several stress development processes in the device manufacturing, such as thermal stress due to mismatch in thermal expansion coefficients among thin film materials used in the device structure and intrinsic stress which occurs during film deposition. To evaluate precise stress fields in actual device structures, the authors have developed stress simulation methods based on finite element analysis, a measurement method for mechanical properties of thin films, and a microscopic stress measurement method. To improve product reliability of silicon devices, it is confirmed that these methods are effective for control or optimization of mechanical stress fields in the device structure. In this paper, the stress evaluation methods are applied to discuss the effect of passivation film stress on the shift in threshold voltage of GaAs FETs.