The combustion characteristic and cycle-to-cycle variations of a DI engine using oxygen-enriched intake air (the volume fraction of oxygen as follows:21%, 23%, 25% and 30%) and water-diesel fuel emulsions (the amount of water was 0％, 10％, 20％and 30％ by volume) were investigated. In-cylinder pressures of 20 combustion cycles and maximum pressures under economic speed and medium load operation were recorded. Consequently, cycle-to-cycle variations of maximum pressures and combustion characteristic were analyzed. The results reveal that, under the conditions of pure diesel, with the increase of oxygen concentration, the maximum cylinder pressures increase, and the cyclic variability of the maximum cylinder pressures and the ignition timing decrease. Using water emulsion diesel, with the increase of volume fraction of water, the ignition timing increases, but it still decreases with the oxygen concentration increasing. When the fraction of water is over 30%, the ignition delay increases dramatically, the in-cylinder burning deteriorates, and the variations of maximum pressure grow.