Over the past few years, drones have become a popular tool for a variety of applications related to nuclear activities, including outdoor and indoor surveys and dose mapping. Drones have been employed by the industry in terms of improving worker safety, saving time and reducing costs. In this study, an unmanned aerial monitoring system (UAMS) was designed and fabricated under its limitation to detect radioactive hotspots. The final goal is to map environmental radioactivity and extract radioactive concentration points; therefore, preliminary experiments were performed to reach a robust detection system and also to study effective flight altitudes that UAMS is able to detect anomalies. UAMS consisted of a detection system including a two-inch sodium iodine crystal, a data-acquisition system and a mini-computer that all were installed under a drone body. One Cs-137 and two Co-60 sources were used for the initial monitoring of UAMS. The results showed that the system is able to detect the sources in the height range of 80 cm to 3 meters.