Abstract: With mechanical product structures and working environment becoming more and more complex, correct evaluation on mechanical system reliability has important practical significance for the product quality evaluation and the whole life cycle management. In this paper, modeling principles as well as research status and problems of the existing quantitative reliability models of mechanical systems are briefly introduced. It is pointed out thatthe research on static system reliability models is relatively mature and the traditional stress strength interference model can be extended to evaluate reliability of mostmechanical components and mechanical systems. However, when the strength of components degenerates, time has to be taken into consideration in reliability analysis of mechanical systems and traditional static reliability models are incapable of analyzing time-dependent reliability of mechanical systems. Therefore, key problems in establishing time-dependent reliability of mechanical systems are described in this paper.