Polymers attract more and more attention as low loss dielectric materials in RF and millimeter wave field, because of their interest properties at high frequencies. In this work, we report a Quasi-Optical free-space method to derive W-band dielectric constant of some common-used polymers including Polymethylmethacrylate, polyvinylchloride, Nylon66, Mylar, polytetra-fluorethylene, polypropylene, high and low density polyethylene, kapton and polyethersulfone. Free-space transmission/ reflection coefficients of the aforementioned materials are measured with a millimeter wave network analyzer. Genetic algorithm (GA) is employed to compute the complex dielectric constant. The results are compared with those in other previously published works.