A parametric equation of state was derived for water vapor in the critical region. It is valid for pressures ranging from 3000 to 4000 psia, specific volumes from 0.8 V/sub c/ to 2 V/sub c/ and temperatures from saturation to 752 degrees. The equation satisfies the boundary conditions at the critical point. The maximum derivation between computed and measured values of pressure of three parts in one thousand coincides approximately with the over-all uncertainty in Pressure-Volume-Temperature measurements. By utilizing certain concepts in "Clausius Theorem of the Virial" the size of water molecule was estimated as 2.61 A from P-V-T data. This is in fair agreements with other estimated values which ranged from two to five Angstroms.