In performing the reliability of a mechanical system, models are formulated to fit the logical structure of its decomposition to subsystems and components. But in many cases the system is of a more complicated nature that it seems difficult. to relate its configuration to a known model. In the presented paper a new technique is introduced to determine the reliability of a complex mechanical system with unknown configuration to its subsystems. The solution is based on the experimental data of testing the system as a whole without a need to any information about how the subsystems are interconnected. Even when the tested sample is only one system it is possible to get an information about the reliability of any one of its subsystems.