Recent technological developments and increased utilization of Unmanned Aircraft Systems (UAS) have widened their application from military operations to also civil and commercial operations. UAS are most beneficial when they can share the whole airspace with manned aircraft. However, integration of UAS into non-segregated airspace is only viable if UAS operations are proved to be safe enough. The concern is that UAS operations could pose a safety problem for other aircraft and persons or property on the ground .The objective of this paper is to develop and apply a safety risk analysis methodology for the risk of collision of an Unmanned Aircraft System with the ground. Such method could support regulators with the setting of UAS safety requirements. It may also be used by applicants (UAS operators and manufacturers) for identification of UAS related hazards, causal factors, and accident scenarios. The method provides insight into the probability of a UAS collision with the ground in relation with safety objectives and requirements.To better understand the differences between manned and unmanned aircraft, UAS performance characteristics are examined. This helps to clarify in which airspace classes the different available UAS may be able to fly. This concerns the broad range of size, various configurations and different performance characteristics (maximum takeoff weight, maximum flight altitude, endurance and maximum speed).Next, causal models are developed for each accident scenario that may result in a collision with the ground. These causal models are represented as Event Sequence Diagrams (ESDs) and Fault Trees (FTs), and provide a logical structure showing how hazards and causal factors could combine to cause a collision with the ground. This approach utilizes the Causal model for Air Transport Safety (CATS) developed for the Dutch Ministry of Transport . Specific UAS related ESDs are added to cover UAS specific hazards that do not exist in manned aircraft operation . Using the newly developed ESDs, a UAS accident probability model is developed for determination of the frequency of occurrence of a UAS accident in relation with safety objectives and requirements.In third party risk analyses, usually Societal risk (to a group) and Individual risk (to individuals at a location) are evaluated. It is recommended to extend the method with an accident location model and an accident consequence model, in order to assess UAS third party risk in terms of these risk metrics.