Summary form only given. Determining the retirement time of aged equipment in power systems has been a challenge in the utility industry for many years. The paper presents a probabilistic analysis approach to making decision on retirement of aged equipment in transmission systems. The basic idea is to quantify and compare the expected damage cost and capital saving due to delaying the retirement of aged equipment. An actual example of an aged underground cable in a regional transmission system of a utility is used to demonstrate the application of the presented approach step by step.