Due to environmental protection efforts, alternative fuels and combustion processes, which can be used to reduce the amount of harmful substances released into the atmosphere, are increasingly widely researched nowadays. In this study, the effect of superheated steam injection on NOx emissions during the combustion of heating oil fuel was investigated. The fuel flow into the combustion chamber, where it mixed with the ambient air in different throttle positions (100%, 70%, 40%, 10%) with different fuel consumption (0.2, 0.4, 0.6, 0.8, 1 kg/h) During the tests, the NOx concentration was measured with an AVL AMA i60 gas analyzer. At each measurement points, a decreasing in NOx was observed with the injection of steam. During the measurements, the temperature of the flame was measured with a high-speed FLIR camera. During the addition of superheated steam, the heat-removing effect of the water could be experienced, which resulted in a decrease in the flame temperature.