Thanks for reply! my question is, under some condition, one use case will execute only part of the other use case. for example, there are two use case, A:1.do a1 2.do a2 3.go to use case B step 2 B:1.do b1 2.do b2 3.do b3 how should I show it in my sequence diagram? Mayby from use case good practice point of view, we should modify our use case so that such situation not happen. Thanks!