This paper aims to examine the effect on performance and environmental-economic indicators of euro diesel-hydrogen dual-fuel combustion in a small turbojet engine suitable for the propulsion system of unmanned air vehicles (UAVs). For this purpose, experimental studies were conducted at seven different engine speeds, in the range of 40,000–100,000 rpm, of a JetCat P80-SE type small turbojet engine with a single-stage radial compressor and an axial turbine on the same shaft. The hydrogen flow rate for each engine speed is changed in 10 l.min−1 increments up to 120 l.min−1, and the euro diesel fuel energy is substituted with hydrogen, resulting in different hydrogen energy fractions (HEFs). The results of the paper show that the maximum reduction in SEC is 9.6% with 40% HEF at 40,000 rpm. In addition, with 15% HEF provided for each engine speed: while the reduction in specific energy consumption (SEC) at 40,000 rpm is 6.5%, at 100,000 rpm the reduction in SEC is 1.2%; the decrease in CO emissions at 40,000 rpm is 33%, while at 100,000 rpm the decrease in CO is 54%; in the range of 40,000–100,000 rpm, the decrease in CO2 emissions changes the range of 16–18%; while the increase in HC emission is 10% at 70,000 rpm, it increases up to 50% at 100,000 rpm; the increase in NOx emissions appearing after 80,000 rpm, is only around 1 ppm; and despite the increase in HC and NOx emissions by the increase of HEFs, the decrease in CO2 and CO emissions leds to a remarkable improvement in environmental-economic indicators.