This paper demonstrates a negative refractive index material composed of an array of circular single split-ring resonators. We discuss in details the design and its features. The reflection and transmission spectra are measured using quasi-optical technique with normal incidence. The measured magnetic resonant frequency that is exited by the electric field is 142 GHz. Moreover, we study the various parameters of the design such as the refractive index, figure of merit, group velocity and phase velocity. The obtained results from the measured data are in good agreement with that retrieved from the simulations.