Abstract: Based on the generalized Huygens-Fresnel diffraction integral,the transformation properties of Gaussian beams passing a titled lens are studied.A closed-form propagation equation is derived and some useful results are obtained.A comparison of tilted-axis Gaussian beams with off-axis Gaussian beams is mode.The beam quality of tilted-axis Gaussian beams is analyzed in terms of M2 factor and the power in the bucket.