Frequencies and damping rate of ion-acoustic waves in the presence of electron-ion collisions are computed by reducing the linearized electron Fokker-Planck equation and cold-ion fluid equations to an eigenvalue equation via moment expansion of the perturbation of electron distribution function. As electrons becomes less collisional,a great number of Sonine modes are needed for convergence to a desired accuracy in the calculation, which may be ascribed to the neglected electron-electron collisions.