With advances in World-Wide Web applications and technologies, research on measurement and modeling of Internet and Web-based information systems has become increasingly important. This paper focuses on continuously monitoring Web traffic by packet sniffing on high-speed links, which is the foundation of analyzing theoretical models of Web characteristics and evolvement. The author presents MOT, a memory online tracing system of Web traffic. MOT parses all packets in memory directly without involving unnecessary I/O operations with magnetic disks to enhance the system performance. Event-driven design pattern and several other techniques are adopted in MOT to overcome the difficulties of buffering huge volume of traffic. MOT provides a fast, accurate and safe way to obtain the source data for many Web-related studies.