Analytical equations of state are presented for fluid mercury in metal, nonmetal, and in metal-nonmetal transition states. Equations of state for metal and nonmetal states are simple in form but the complexities of the transition state leads to a complex fourth-order equation. The interatomic potential function used to describe the metal state has a hard repulsion wall, and that of the nonmetal state is the same as the potential function of the nonpolar fluid with induced dipole intermolecular interaction. Metal-nonmetal transition occurs in the liquid density range 11-8 g/cm(3), and a density-dependent interaction potential function, which gradually changes from a pure metal interaction to a nonmetal interaction in the transition region, is used. Well-depth and the position of potential minimum are presented as temperature dependent quantities; their calculated values for the metal state are typically within 5.0% and 0.33% of the experimental value, respectively. The calculated well-depth for the nonmetal state is smaller than the experimental value indicating the effect of high pressure PrhoT data used, which pushes a pair of mercury atom further together into the repulsion side. In the transition region, calculated well-depths are 2-3 orders of magnitude larger than those for the metal state and contain a sharp rising edge and a steep falling, having a singularity characteristic of phase transition.