The sharing of airspace by both manned aircraft and Unmanned Air Vehicles (UAVs) has been an issue for decades. In recent years the fast growth of civil UAV activities has led to the concern for their threat to aviation safety. Hence, it is becoming more important to establish regulations for UAV operation, in which the aviation regulations must prescribe not only the required performance and equipment of UAVs, but also the allowable airspace for UAV activities. Regarding the airspace monopolized by manned aircraft in Taiwan, this paper is intended to focus on the study of the allowable airspace demarcated by safety issues. For this reason,
parameters involved will cover Mean Time Between Failures (MTBF), terrain controlled airspace and restricted airspace. Analyzing the capacity in limited airspace is also included from efficiency and effectiveness aspects.