This article proposes a ray-tracing and physical-optics (RT-PO) model that allows for an accurate and time-efficient evaluation of planar Mikaelian lens antennas implemented by parallel plate waveguides. With an intrinsic flat shape and axis-symmetry of refractive-index distribution characteristic, the planar Mikaelian lens antennas are easy to fabricate and integrate to standard planar feeds. A numerical computation of the ray paths based on the Snell’s law gives a description of the phase of the electric field in the lens aperture, while the ray-tube power conservation theory is employed to evaluate the amplitude. The field equivalence principle is then used to calculate the far field of the lens antenna. The information of far-field directivity, gain, and dielectric efficiency is further obtained, considering a small loss in the dielectric materials. Our approach is validated by comparing the results of a particular Mikaelian lens antenna with those computed using a commercial full-wave simulator, demonstrating high accuracy and significant reduction in computation resources and times.