Single split-ring resonators has a great interest due to their refractive index facility. In this paper we provide a comprehensive study, supported with experimental data, that addresses the e{\textregistered}ect of the type of the substrate and the geometric parameters on the resonant frequency and the quality of the design. The measurements are carried out using quasi-optical technique in mm-wave frequency range. We also provide recommendations concerning the main factors that should be considered in designing split open ring resonators at mm-wave frequency range.