There is provided a method for quantitatively evaluating properties of a graphite structure of a gray cast iron based on a number and a thick and thin degree of graphite components in the structure. In particular, only non-spherical graphite pieces having an average size of 5 .mu.m or more are extracted from a preprocessed image of the graphite structure, and counted (Step S5 of FIG. 3). Further, only graphite pieces having a maximum length of 50 .mu.m or more and less than 150 .mu.m are selected therefrom, and a length and an area of each selected graphite piece are measured (Step S6). An area of an assumptive representative graphite piece having a maximum length (a maximum size) of 100 .mu.m is calculated from these data, and divided by the length 100 .mu.m, to obtain the thick and thin degree (Step S7). The thick and thin degree is shown with the number of the graphite pieces (Step S8).