The equations of state and ionization degrees of helium, neon, argon, krypton, and xenon in the region of ionization have been presented based on the Saha equation with the Debye-Hückel correction. The calculated results were compared with the experimental data and other calculations and the reliability of this model has been validated. The compressibility and the regularity of ionization were discussed by the calculated isotherms and shock compression curves of the rare gases. The results indicate that xenon has better compression resistance among the gases cousidered.