UML sequence diagrams are widely used during requirements analysis and design for specifying the expected message exchanges among a set of objects in various scenarios for the program to perform a certain task. In this paper, we present the behavior view diagrams, a type of extended sequence diagrams, to facilitate execution monitoring during debugging. Using a behavior view diagram, software developers can precisely specify the runtime objects whose behaviors will be monitored during debugging. Software developers can also specify the important message exchanges to be observed among these objects during the progress of various scenarios, and may further define the monitoring actions to be performed for inspecting the program state when a message exchange is observed. We also present a debugger that can automatically monitor the program execution using the information specified in a behavior view diagram. Through this monitoring, the debugger can not only check whether the scenarios are progressed as intended, but also check whether the actions performed by the program have the the desired effects on the program states. Therefore, it will be useful for detecting and localizing bugs.