Temperature sensitivity is an important measure in the evaluation of radiometers. However, it is not directly determined by traditional methods of radiometer performance evaluation. Instead, beam efficiency is commonly evaluated to determine the accuracy of a radiometer antenna. This paper introduces a new parameter, the residue beam efficiency ratio, which relates directly to the relative error of temperature measurement and overcomes the inaccuracies and difficulties associated with beam efficiency. Also discussed is an optimization method which uses the minimum residue beam efficiency ratio to improve the temperature resolution and to reduce the antenna aperture diameter. Although the concept may be extended, the scope of this paper is limited to large-reflector antennas for space-based remote sensing of the Earth. ¿ American Geophysical Union 1995