With the occurrence of new applications such as Voice over IP (VoIP) and multimedia conference, there is an ongoing discussion about realizing QoS in the Internet today. Because of its potential scalability in support of QoS guarantees, the Differentiated Service (DiffServ) architecture with aggregate packet scheduling has recently attracted much attention in the networking community as a feasible solution for providing Internet QoS. Thus, it is important to understand delay bound of an individual flow in the DiffServ architecture in order to provide delay-sensitive applications. In this paper, we study, via both analysis and simulation, the deterministic bound on edge-to-edge delay of a flow in a DiffServ network domain with FIFO aggregation and a class-based Latency Rate (LR) server that provides guaranteed performance with rate reservation for a traffic class. We derive edge-to-edge delay bound for a single flow as a function of allocated service rate for a traffic class, token bucket parameters adopted for flows at the network ingress, and information about joining and leaving flows. We compare the obtained delay bound with previous works using analytic results, and then conduct simulation to confirm the results. The derived bound is less than that of previous studies in all cases.