Increased size and complexity of software requires better methods for different activities in the software development lifecycle. Quality assurance of software is primarily done by means of testing, an activity that faces constraints of both time and resources. Hence, there is need to test effectively within the constraints in order to maximize throughput i.e. rate of fault detection, coverage, etc. Test case prioritization involves techniques aimed at finding the best prioritized test suite. In this paper, we propose a prioritization technique based on UML activity diagrams. The constructs of an activity diagram are used to prioritize scenarios. Preliminary results obtained on a case-study indicate that the technique is effective in extracting the critical scenarios from the activity diagram.