A new network measurement primitive was recently proposed, known as long duration flows (LDF). LDF deserves special attention for network management and security monitoring. This kind of traffic appears periodically and persistently through a long period, but its total amount of traffic is not necessarily large. This feature makes detection difficult especially when the resources of detection system are limited or the detection should cover high-speed networks. In this paper, we propose a new lightweight data structure and streaming algorithm to detect such traffic.