The radiation efficiency of nano antennas is a critical parameter determining their suitability for various applications. To that end, the radiation efficiency of a single antenna is directly quantified using far-field measurements of the reflectance of an array comprising identical nano-antennas. Excellent agreement in terms of resonance frequencies, optical bandwidth, and scattering efficiency is found between the results of experimental measurements and finite element-based numerical analysis of scattering from Au nano-antenna arrays. Extremely high overall radiation efficiencies, exceeding 82%, are obtained. The high efficiency can facilitate the employment of such nano-antennas for applications in imaging, spectroscopy, and solar energy harvesting.