In this paper, a new free-space measurement setup at millimeter waves for material characterization is presented. Using specific Gaussian optics lens antennas and a thru, reflect, and line calibration, the setup provides the free-space four S-parameters over the W-band of planar dielectric slabs without time-domain gating. An efficient optimization procedure is implemented to extract complex permittivity from the four S-parameters of homogeneous dielectric materials. Non homogeneous materials can also be tested, and measurements are presented. Very good agreement is observed between simulated and measured four S-parameters of various dielectric plates. Thanks to this new specific calibration and measurement procedure, automation of the test bench is easily achieved.