Supercritical water oxidation (SCWO) of dimethylamine (DMA) and trimethylamine (TMA) was conducted at 400–450°C and 25 MPa in the presence of excess O₂. An apparent induction time was experimentally observed only in the oxidation of DMA. Monomethylamine (MMA) was the common major product in both oxidations of DMA and TMA in spite of the different numbers of substituted methyl groups, suggesting that the oxidations of two amines were via different reaction mechanisms. The SCWO of DMA was considered as a radical chain reaction that proceeded via such a peroxyl radical as CH₃NHCH₂O₂, leading to the appearance of the induction time. In contrast, the SCWO of TMA mainly proceed via formation of N-methyl methanimine (CH₂=NCH₃) followed by its hydrolysis to form MMA and formaldehyde. We proposed global reaction mechanisms that can reasonably explain the experimental observations in DMA and TMA oxidation in supercritical water.