In this study, we investigated the effects of Air Fine Bubble A-heavy oil (AFBA) on fuel consumption of and NOx emissions from high-speed diesel engines. Air Fine Bubbles (AFBs) from a pressurized dissolution-type Fine Bubble Mixing Unit (FBMU) were mixed into fuel to measure their size distribution. The results found that changing the amount of air supplied to the FBMU could control the number, diameter and void fraction of AFBs. When conducting an experiment with a high-speed diesel engine, the use of AFBA reduced fuel consumption by up to 6.1 % and by 2.0 % in the load average and cut NOx emissions by up to 19.6 % and by 10.5 % in the load average. These reductions were likely caused by the promotion of premixed-like combustion and lower combustion temperatures triggered by AFBA. The effects of reductions in fuel consumption and NOx emissions were more evident when void fractions increased. Simulation of spray combustion in a cylinder was performed to examine the impact of AFBs-induced atomization of spray droplets on the promotion of premixed-like combustion. The results showed that premixed-like combustion was promoted by AFBA due to not only the atomization of the spray droplets, but also precipitating air at the local pressure drop region in the fuel injection nozzle that promoted cavitation as bubble nuclei.