Aqueous potassium formate solution is regarded as a potential working fluid of an absorption heat transformer (AHT) because of its noncorrosive and biodegradable characteristics. The vapor pressure of working fluids is an important property for application in an absorption cycle. However, current experimental data are mostly limited to temperature and concentration ranges that are suitable for a dehumidification system. Thus, more accurate data of vapor pressure are anticipated. In this paper, the equilibrium vapor pressures of aqueous HCOOK solution were measured with a static method in the temperature range from 323.15 to 373.15 K. The mass concentration of HCOOK varied from 10 to 70%. The measured data were compared with the estimated values of a referenced chemical engineering property database (Aspen Plus). A relatively large mean absolute relative deviation (MARD) of 13.27% was found. The data were then correlated with a modified Antoine equation, and the MARD was reduced to 0.45%. The thermodynamic performance of aqueous HCOOK solution for AHT was estimated for a typical cycle.