Among the problems encountered in rotordynamics, the phenomena of instability generally due to the hydrodynamic bearings and interns damping, the gyroscopic effect due to the discs and shafts, the excitations due to the unbalance as well as the nonlinear phenomena related on the bearings which support the rotor and the elements carried by the rotor. The objective of this paper is to investigate the effects of damping and rigidity of hydrodynamic bearing on the stability of a Lalanne-Ferraris rotor. This study was conducted with respect to the stability criterion seen in a natural frequencies’ equation. The process begins with the establishment of the characteristics of rotor elements. This is to assess the expressions of the kinetic and deformation energies, as well as the corresponding virtual work for rotor components: Disk, shaft, unbalance and hydrodynamic bearing. The Rayleigh-Ritz method and Lagrange’s equations were used to determine the equations of motion. A computational program in FORTRAN is elaborated to solve the characteristic equation in free vibration. The roots are pairs of complex conjugate quantities. In forced vibration, the resolution of the linear algebraic system is conducted by the Gauss-Jordan direct method. Also a computational program in FORTRAN is elaborated. A numerical example of Lalanne and Ferraris rotor is computed. According to the results presented herein, the presence of damping for certain values of rigidities was found to be a possible source for instability of the rotor at defined speeds.