The term performance of an aircraft means the ability of an aircraft to to have the mosta efficient characteristics under certain conditions (altitude, speed, mass) in form of flight endurance, range, fuel consumption, length required for take off and landing, etc. At the beginning of this paper are presented performance of the aircraft Diamond Katana DV20 which are found in it′s manual. The following section of the paper describe the model of propulsion, used in aircraft, along with aerodynamic model form the work of Petar Papoči, to calculate performance of this aircraft in the cruise and climb phase. At the end of this paper, the data obtained in this paper will be compared with the published performance data. In accordance with theoretical calculation the given figure for maximum flight endurance is 4,9 h, maximum cruising Zagreb, 2019. speed is 236km/h, maximum lifting speed is 4,8 m/s, and the speed for maximum velocity is 100km/h. At the end of this paper the data acquired during theoretical research was compared to the published performance data of the aircraft. Based on the comparison between the theoretically obtained data and the declared performance data it can be concluded that deviations in data are minimal and acceptable.