This work presents a design approach for optically transparent low-loss frequency selective surfaces (FSSs) based on the simultaneous optimization of a conductive layer, dielectric layer, and fabrication process. Three bandpass FSSs working in millimeter-wave bands with low insertion losses are investigated and fabricated based on low-loss Rogers RT5870 and cyclic olefin copolymer (COC) substrates. The measured results of the RT5870 based FSS (case 1) and COC based FSS (case 2) indicate that the COC substrate possesses lower dielectric loss tangent at the K band. Optically transparent FSS (case 3) is then investigated and optimized simultaneously on the selection of dielectric substrate (dielectric loss and optical transparency), conductive circuit structure (conductor loss and optical transparency), as well as the fabrication process in terms of insertion loss, optical transparency, frequency selectivity, cost, and ease of fabrication. The proposed case 3 optically transparent FSS based on COC substrate and metallic mesh owns lower insertion loss (0.62 dB) than that of the state-of-the-art FSSs in literature as well as the fabricated non-transparent case 1 and case 2 FSSs. The COC based optically transparent FSS has the merits of high optical transparency, low insertion loss, good frequency selectivity, low profile, ease of fabrication process, and low cost, showing great potential in millimeter-wave applications.