Since few years, it observed a rapid growth in interest on unmanned aerial vehicles (UAV) in many applications. The UAVs are observed as an entertaining equipment for kids, a sophisticated tool in agriculture, archaeologyor terrain scanning. There are also military applications of these flying objects in various types of missions. Everything makes that we can observe several configurations of UAVs, which can tackle specific requirements. The whole variety of UAVs application need a wide range of propulsion systems. In presented paper tha authors tried to present and evaluate specific parameters of engines used in modern UAVs. There are described electric motors, piston engines and turbine engines, which produce from about 18 W to 900 kW of power. That is very wide range of power, what we can assume that the engines are incomparable, but after obtaining the power to weight ratio, it can be seen that in most cases, the power to weight ratio keeps the values at about 0.1 to 0.15 kW/kg. In some specific cases this ratio has different values.