Water lubrication has been attracting attention for environment-friendly society due to low CO2 emission. Furthermore, carbon-based materials such as diamond-like carbon (DLC) show the low friction properties in water lubrication due to the oxidation reaction on the surface in pre-sliding. However, the influence of oxidation reactions on low friction mechanism is still unclear. In this study, we clarify the structure change of DLC with the oxidation reaction in the pre-sliding using first-principles calculation, which suggests the low friction mechanism of DLC in water lubrication. The results show the structure change from sp3 carbon (Csp3) to sp2 carbon (Csp2) by the oxidation reaction on the surface. Furthermore, the Csp2 rich surface in water lubrication indicates the smooth sliding. We suggest that the structure change from Csp3 to Csp2 would affect low friction properties of DLC in water lubrication.