Abstract: The effect of Earth’s oblateness in the inversion of GPS occultation data and corresponding correcting method are studied. By using observed CHAMP orbit ephemeredes and MSISE-90 dry air model and regarding Earth as a sphere and an ellipsoid respectively, excess phase delays are simulated and the simulated data are retrieved under different schemes. The comparisons between the Inverted temperature profiles and the model temperature profiles show that when inverting observed data, we will get temperature results with large errors if the effect of Earth’s oblateness is omitted. The correction method is tested useful because the temperature errors decreased obviously with this method.