The stability of journal bearings is analyzed by the perturbation method, clarifying a treatment of a time derivative term of the lubricant density ratio in the Reynolds-like equation based on the mass-conservative cavitation model. The stability map by the perturbation analysis is verified with the nonlinear transient analysis. In ε<0.1, the stability by the mass-conservative cavitation model was always unstable. In 0.1≦ε, the model gave the higher stability boundary than the non-mass-conservative half-Sommerfeld model. The choice of the cavitation pressure was significant in the bearing stability at lower eccentricity.