The reliability of a mechanical system is not simply determined by the reliabilities of components composing the system. For a specified system configuration, system reliability depends on not only component reliability, but also the degree of the dependence among component failures, which can be characterized by load roughness in an extent. In the same way, system complexity depends on not only the number of components that the system contains but also the dependence among component failures. This paper analyzes the main factors that affect the relationship between system reliability and component reliabilities in term of component failure dependence. The factors include load uncertainty, component strength uncertainty, and load roughness as well. Traditional parameter describing system complexity is modified, new parameter to describe component complexity is defined. These parameters play important rule for system reliability design or system reliability allocation.