The research aimed to analyse the influence of diesel oil temperature on NOx emission level. The tests were carried out on a test stand equipped with a 9.5 kW multi-fuel compression-ignition engine. The stand is an experimental cogeneration unit discharging the produced energy into the power grid. The measurements were carried out under the D1 test procedure provided by ISO 8178-4 for testing engines operating at a constant speed. As a result of statistical analysis of the results obtained, significant differences were found in the u specific values of nitrogen oxides emission gained for particular phases of D1 tests, when the engine was fed with diesel fuel of different temperatures. The results obtained confirmed the possibility of limiting the specific emission of NOx only when the engine is running at 75% of its nominal torque. An increase in NOx emissions was recorded for the remaining loads.