The information indices of the interpretation quality evaluation are usually regarded as the most important basis for measuring the faithfulness of translation. The information units, namely, the units of measurement of the indices, are essentials of evaluation. Nevertheless, up to now, there has never appeared any systematic exposition concerning the identification as well as the definition of this kind of units. This paper will, on the strength of the exposition of the relevant subjects, discuss the information units of the interpretation quality evaluation in terms of the essence of interpretation, the distinction between the units of measurement and the units of translation, the definition of information units and the relationship between information units and discourse analysis. This paper, with the results of the theoretical research serving as an guidance for interpretation evaluation practice, attempts to explore a set of original evaluation methods which are to be objective, scientific, easy-to-handle and feasible.