The coupling between the emitted energy and the acoustic energy in a thermal system, e.g. combustor, may result in system instability accompanying severe vibration. In this study, the limit curve for thermo-acoustic instability is numerically obtained by using the CFD-CAA combined method. The response function for pulsating release of heat in the Rijke tube and the distribution of mean density in the tube are computed accurately by the CFD analysis and used as inputs to CAA predictions. To verify the accuracy of the prediction method, the thermo-acoustics inside the horizontal, one meter long Rijke tube of square cross-section, which has the heating part at the location of one quarter length, is simulated. The predicted instability curve agrees well with the experimental one except the low flow-rate region of $Re_{Do}$ < 1.