One method suggested in functional safety standards for quantitative analysis of systems is Markov processes of which Semi-Markov processes (SMPs) is an extension allowing more accurate models of the systems of interest. The paper suggests state transition diagrams modeled in Stateflow for modeling systems as SMPs since it will ease the adoption of the method in the automotive industry. Compared to the method of fault trees, which is frequently applied in industry today, SMPs have a significant advantage for some systems given their ability to explicitly model time-dependent aspects of the system. SMP-tool is suggested as a tool support for quantitative analysis of state transition diagrams modeled in Stateflow