This paper addresses the measurement of compact flat lens antenna unit cell in the X-band frequency range using waveguides. A set of sample unit cells based on aperture-coupled concept were fabricated and measured. Two standard WR-90 waveguides and vector network analyzer are used to measure the fabricated elements. The transmission and reflection loss performances of the measured sample unit cells are demonstrated and validated with CST software simulations. The experimental results obtained are promising and indorses that waveguide simulators are precise and cost-effective method of measuring the scattering parameters of flat lens antenna unit cells.