Abstract: The light transmissivity “T” of an object is the ratio of transmission intensity to incident intensity. Only if objects have the same thickness, the transparency of different objects can be compared with. The unit transmissivity “t” of an object with 1 mm has been introduced in this paper as a fundamental unit in comparing the transparency of the objects. The unit transmissivity “t” can be represented by t=n \barT, where n is the thickness of the object; T is the transmissivity. Both n and T can be obtained by measuring the object. Based on the qualitative grading of jadeite jade studied by predecessors, the quantitative measuring and grading method are developed.