Although it is reported that ash is generated from combustion of a lubricant oil, there are few studies on the kinetic mechanism of the ash under the combustion conditions. In this study, the reaction pathway for ash formed from the oxidation of Ca-based lubricant oil additives in a flow reactor was investigated. The quantitative analysis of the ash components formed from the oxidation of calcium sulfonate and calcium salicylate in the temperature range 773 – 1373 K was performed through high-performance liquid chromatography. It is found that the ash components were mainly oxides of typical metal components contained in lubricant oil additives. Based on the analysis of the temperature-dependent products, reaction mechanism for ash formation was evaluated.