Hydrothermal reaction of formic acid has been studied by varying the following conditions: temperature (275 to 350degreesC), concentration of formic acid (0.1 to 1 M), addition of HCl, and presence of metal alloy powders. In hot water, formic acid decomposes to CO and CO2. The product ratio of CO to CO2 strongly depends on the conditions. At higher concentrations and lower temperatures, the yield of CO gets larger. In most of the conditions studied here, the main product is not CO2 but CO in contrast to previous works. Acid enhances the CO production and metal powders catalyze the decarboxylation pathway.