Abstract: By using iTEM image processing software, the scale of the metallographic picture can be changed to make the nominal magnification of the image meet the standard requirements. By using the measurement function of iTEM software, a circle can be made in the picture, and then the software will automatically calculate the area of the circle under the standard magnification. By clicking on the point count measurement options, the number of grains inside the circle and intercrossed with the circle can be calculated quickly respectively, and then the number of grains per unit area can be obtained. Then according to the standard of ASTM E112-2013 in which the grain size number is defined, the grain size number of the sample can be calculated. The results show that the grain size number calculated by using iTEM software was in good agreement with the standard values of the sample.