Characteristic grain size, G c, is defined in the present paper. It indicates an intrinsic size of grain for a given system, and has the following advantages over the average grain size which is based on a statistical mean value of the linear intercept length of grains: (1) The characteristic grain size G c has clear and determined geometric meaning, i. e. G c refers to two times of the area-weighted average height of the surface elements to the centroid of grain or the average length of the chord through the centroid of grain; (2) The value of G c can be directly obtained without introducing any correcting factors such as grain shape or grain size distribution factors. Generally, G c is larger than the average grain size G when the Heyn intercept method is adopted for the grain size measurement.