An acidic two-step hydrothermal process to improve acetic acid production was investigated. In the first-step reaction, levulinic acid is formed in the presence of an acid catalyst. In the second-step reaction, the levulinic acid is converted into acetic acid via oxidation with newly supplied oxygen. The acetic acid yield from this two-step process was much higher than that without the acid catalysts in the first step. The acetic acid yield reached 23% with a purity of 47% from glucose and 26% with a purity of 52% from fructose. These yields were achieved at a reaction temperature of 523 K, a reaction time of 300 s, and pH 0.5 with HCl as an acid catalyst for the first step and at a reaction temperature of 523 K, a reaction time of 60 s, and an oxygen supply of 100% for the second step.