The order-disorder phase transition for KH2PO4 (KDP) and ice VIII is studied in terms of the dipole-hydrogen coupling model. The effective interactions between hydrogens are obtained by averaging out the dipole’s degrees of freedom. It is found that the bonding energy of ice VIII obtained in the present theory compares with the hydrogen bond energy determined by Whalley. The isotope effect is explained in the difference of bonding energies between KDP and DKDP, without the tunneling energy. It is shown that the energy levels excited from the states with stable hydrogen bond in ice VIII are considerably higher than those in KDP, and the concentration of ionic defect in ice VIII is much smaller than that in KDP. Our result can explain the well-known fact that the ice rule condition is conserved in ice VIII, on the contrary broken in KDP.