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A method of assessing the quality of service (QoS) of TCP/IP-based video streams for real environments is proposed. To investigate the cause of video quality degradation and to observe the video QoS level at user clients, a method for monitoring the QoS-related parameters in both the transport and application layers for the target system is employed. To detect the cause of QoS degradation, the authors focus on the number of packet retransmissions and the transmission delay of packets observed in the transport layer. Also, for measuring the QoS level, the bit rate of video streams and buffer usage rates in the application layer are the focus. Based on measurement using an MPEG-4 video system in a real environment, it is confirmed that the buffer usage rate in the application layer tended to decrease before the detection of sudden decreases in the bit rate of video streams, and that underflow events where the buffer usage rate dropped to less than 30-50% tended to cause QoS deterioration of video applications. The proposed method deals with QoS-related parameters for TCP/IP-based video streams so it is possible to efficiently check the reasons for and the patterns of the QoS degradation.