Various applications provide convenient services in the Internet. As these applications are widely used, it is important that the quality of these services (QoS), such as the response time and throughput, be high. Several ways to measure network performance have been developed. However, QoS management is still difficult because of the lack of effective tools to evaluate network system usability. The goal of our study was to develop a new QoS measurement system for network system usability. In our approach, we measure system usability and performance through mechanisms installed in the clients. The purpose of our system is to make clear the behavior of the client applications, and this allows us to measure system performance and system usability. In this paper, we describe the fundamental concept behind client observation and its performance indices, and the design and implementation of a performance evaluation system implemented through observation. In addition, we confirm the effectiveness of our system through experiments.