<p>Ultrafine bubbles (UFBs) are microbubbles with a diameter of 1 μm or less. In recent years, many reports have shown that grinding performance and machining accuracy can be improved by using coolant containing UFB (UFB coolant) as the coolant in the grinding process. However, the mechanism by which this effect occurs has not been clarified. Existing studies consider that UFB coolant has better wettability than normal coolant, which changes grinding performance. If the inclusion of UFB changes the liquid properties of the coolant, then grinding with UFB coolant should be effective for any work material. However, there are cases where the effect of UFB is not obtained. Therefore, wettability is not considered to be the main cause of the UFB effect. As a general characteristic of UFB, it is known that OH radicals are generated when UFB collapses, which is thought to promote oxidation of the mating material. This study examines and discusses the mechanism of UFB effect generation in grinding from the viewpoint of oxidation of the work material surface. In the case of UFB water, the oxidation state of the work material surface was found to differ depending on the gas type. The composition of the oxides formed also differed depending on the gas type. Furthermore, friction and wear tests simulating the grinding process were conducted, and it was found that the friction coefficient varied depending on the type of gas in the UFB coolant, suggesting that the oxidation of the work material surface by UFB affects the friction and wear properties, resulting in a change in grinding properties.</p>