In this paper, I present a method for the evaluation of the quality of translated text, namely, a translation ability index, which shows the relative position of the translation ability of a Machine Translation (MT) system on a measurement scale. The measurements are made by an analysis ratio which is able to make absolute measurements and a conversion and magnitude scale (CGMS) which indicates the mutual relation of the machine translated text to the text translated by the professional human translator. The translation ability index in this work has been confirmed by the evaluation of two MT systems. This is effective as a clear explanation of this work.