Reliability is one of the major concerns for software engineers. The increasing size of software systems and their inherent complexity - which is essentially related to the intricate interdependencies among many heterogeneous components - pose serious difficulties to its assessment and assurance. The actual system runtime behavior is difficult to forecast during the development phase, and just relying upon sound design and testing techniques is often not sufficient to deliver highly reliable systems. In order to guarantee high reliability, system behavior needs to be monitored at runtime and its reliability needs to be periodically estimated during operation, taking into account both structural/static and behavioral/dynamic information. In this paper, we propose an online reliability monitoring approach, which combines static reliability modeling and dynamic analysis to periodically evaluate system reliability trend during operation. Its usage is illustrated by a prototype implementation and a case- study.