A thermodynamic free energy function in the form of an eighth-order polynomial has been developed for bulk potassium niobate KNbO3, based on the Landau-Ginsburg-Devonshire phenomenological theory. The obtained free energy function successfully models the ferroelectricsingle domain intrinsic properties of KNbO3, including the three sequential phase transition temperatures as well as polarizations, dielectric constants, and lattice constants along the whole ferroelectric temperature range. It is applicable to explaining and predicting the extrinsic contributions such as hydrostatic pressure on the bulks properties and substrate constraints on the thin films properties.