This paper presents a case study on how to apply ITIL availability management best practice to assess the end-to-end service availability in a tiered web service delivery environment. It also helps pinpoint the road blocks to the required service level and provide cost-effective solution for improved service delivery. An overview of basic ITIL availability management concept is provided along with the introduction to two key parameters, mean time between failure (MTBF) for reliability and mean time to repair (MTTR) for maintainability. Then a block diagram modeling approach for the overall information system availability assessment is presented. The block diagrams are used to describe the interrelationship among the components or subsystems to define a system model. The ITIL best practice approaches for availability management are discussed in detail in the case study. It demonstrates that the overall service availability is affected by the availability of information systems as well as applications and operational processes. The modeling approach is capable of gaining visibility on root cause of the service availability degradation, identifying risks in the existing information system, and providing support to future application development and infrastructure upgrade for high availability business services.