On the basis of the Gaussian beam assumption and the diffraction theory of light, we studied the optical limiting properties of the refractive optical limiting devices based on self-focusing and self-defocusing, and examined the effects of the location of the nonlinear medium and the waist size of the incident Gaussian beam on the operating performances of the refractive optical limiters. Our numerical results show that the optical limiting curves of the refractive optical limiters assume a vibrating decaying waveform. If the convergence of the incident beam is poor, the distance between the sample cell and the focal plane must be slightly smaller, but if the convergence of the beam is good, the distance between the cell and the focal plane must be slightly larger. For a self-focusing (self-defocusing) optical limiter, the optimum optical limiting effect can be obtained when the nonlinear medium is positioned at a certain location within half a Rayleigh range in the left (right) side of the focal plane.