We present a method to identify stakeholders and their preferences about non-functional requirements (NFR) by using use case diagrams of existing systems. We focus on the changes about NFR because such changes help stakeholders to identify their preferences. Comparing different use case diagrams of the same domain helps us to find the changes that can occur. We utilize the Goal-Question-Metrics (GQM) method to identify variables that characterize NFR. Thus, we can systematically represent changes about NFR using the variables. The use cases that represent system interactions help us to bridge the gap between goals and metrics (variables). Thus, we can easily construct measurable NFR. In order to illustrate and evaluate our method, we applied our method to an application domain of the Mail User Agent (MUA) system.