Two types of photo-induced gratings, birefringence and surface relief gratings, were formed on an azo polymer film when two orthogonal circularly polarized writing beams irradiated the film. All the diffracted beams from the gratings were measured during irradiation and the surface relief height inscribed on the film was measured by using atomic force microscope (AFM) after the irradiation. The transmitted beam intensities were calculated using a Jones matrix for two gratings. The phase differences for the two gratings were separately determined by fitting the experimental results to the theory. Next, the changes of the phase differences of two gratings were measured in-situ using an exact equations. The results from this method agreed with previous one. The phase difference between two gratings was measured. The phase difference between two gratings converged on about 45 degrees in our case. Diffraction efficiency measurement and analysis revealed that the two gratings were independent of each other and that the mechanisms for the two gratings were different.