In the context of component-based software development, this paper proposes an approach to transforming UML diagrams of software architecture to Markov chain for the quantitative evaluation of reliability. Based on the component-based software architecture, it utilizes four types of UML diagrams: use case, sequence, activity and component diagrams, extending them and annotating them with reliability related attributes. Then, the diagrams are transformed into a Markov chain based analysis model by constructing an intermediate model called Component Transition Graph (CTG). Result of this transformation can be directly used in the existing analysis methods to predict software reliability, which facilitates the analysis task of software designer.