Abstract: To determine the iteration order is a critical problem in the agile development,and it is the foundation of agile development process．There are a lot of literatures to do a lot of work in this area．But the iteration order is based on the value of functional groups in many literatures．This iteration order based on a single indicator will often be unexpected consequences．The value of the functional groups is mostly determined by the qualitative approach,and it is difficult to using the quantitative methods to realize it．This paper put forward that we can use UML(Unified Modeling Language) use case diagram and sequence diagram to calculate the use probability and risk degree of use cases representing each system functional,to solve the problems about the study of iterative sequence in the agile development process．And based on the use probability and risk degree of cases,we can determine iterative sequence in the agile development process with the method of Probability and Statistics and Fuzzy Decision-making Method for opinion concentrated.

Key words: UML,Use case,Risk,Value,Iteration order