Sewage sludge is a significant by-product from wastewater treatment plants but is potentially hazardous due to its pathogenic or harmful contents, such as toxic metals. Subcritical water wet oxidation (SCWO) is one method of hydrothermal processing which has recently been used in research and industry for treating sludge. This study investigated the effect of SCWO on metals such as Al, Cu, Fe, and Zn in sludge, including the content of metals in the liquid or solid phase after SCWO and their stability distributions according to Tessier's method. During SCWO, most metals were significantly accumulated and stabilised in the solid phase with very limited leaching. The temperature was found to be the most significant factor for metals immobilisation, followed by reaction time, while pH had negligible effect. The optimal conditions of SCWO on metals were found at 240 °C, 60 min, pH 5 in this study.