Electrical and reliability characteristics of dielectric materials are commonly studied under electric fields without considering mechanical stress effect. Electrical and reliability characteristics of silicon dioxide (SiO₂) and porous low-k dielectric (p-low-k) films under mechanical-electrical compounded field effect using metal-insulator-semiconductor (MIS) structures were compared in this study. Under mechanical stress in both tension and compression, the increased capacitance and leakage current, and the reduced breakdown field were observed for both SiO₂ and p-low-k films. P-low-k films displayed a larger degradation under mechanical stress. After mechanical unloading, electrical properties and reliability cannot be recovered. Additionally, the tensile and compressive stresses caused a similar degrading effect on a p-low-k film. In case of SiO₂ films, a larger degradation was detected under compressive stress. This investigation demonstrates mechanical stress can influence the electrical characteristics and reliability of dielectric and are needed to pay more attention in this topic.