Understanding the end-to-end packet dela y dynamic of the Internet is of crucial importance since it directly affects the QoS (Quality of Services) of realtime services, and it enables us to design an efficient congestion control mechanism. In this paper, we measure the round-trip time, and build a mathematical model representing its dynamics using system identification. We first measure, as the input and output data for system identification,the pac ket inter-departure time from a source host and the corresponding round-trip time measured by the source host. ICMP (Internet Control Message Protocol) is utilized to measure the round-trip time for each packet. We next model the network, seen by a sp ecific source host, as a dynamic SISO (Single-Input and Single-Output) system. Using measurement results obtained from three different net-w orkconfigurations, we investigate how accurately the round-trip time dynamics of the Internet can be modeled with the system identification.