Simulation has long been applied to enhance training and preparation. Nowadays, however, simulation is being used as a technological enabler to support all phases of a project lifecycle, including research, analysis, and acquisition. With the ability to virtually simulate systems, processes, communications and the interoperability of each in a simulation environment, simulation helps reduce the cost and risk associated with all phases of a project. UAV (Unmanned Aerial Vehicle) performs various kinds of missions such as mobile tactical reconnaissance, surveillance, law enforcement, search and rescue, land management, environmental monitoring, disaster management. In order to realize all functionalities of the UAV, the software part becomes very complex real-time system expected to execute real-time tasks concurrently. This paper reports experiences of designing and developing simulator for lightweight UAV purpose-built for mediumscale reconnaissance and surveillance missions in civil area. The overall system architecture and implementation are described.