The P-V-T relations for polar substances are studied by using a partition function. The attraction effects are divided into polar and nonpolar term
and a polar partition expression with intermolecular potential function is derived
to modify Van der Waals equation of state and obtain a correlation for attraction parameter"a". Polar substances are correlated with critical compressibility factor as well as reduced dipole moment. The calculated vapor pressure and heat of vaporization of 45 kinds of polar substances are satisfactory. A modified