To make the objective standard of nuclear size ingrading nuclear pleomorphism of invasive ductal carcinoma of the breast, we measured maximal nuclear diameter of tumor cells on imprint cytology slides and histologic sections from 65 cases by using computer-based image analysis system(Optimas 6.0). The maximal diameter of red blood cells were also measured to evaluate the ratio of maximal nuclear diameter of tumor cells to maximal diameter of red blood cells. The mean values of maximal nuclear diameter of tumor cells on imprint cytology slides and histologic sections were $7.56{mu}m,;7.53{mu}m$ in nuclear grade 1, $8.92{pm}0.98{mu}m,;9.02{pm}0.74{mu}m$ in nuclear grade 2, and $12.90{pm}1.47{mu}m,;12.44{pm}1.41{mu}m$ in nuclear grade 3, respectively. There were no significant differences between values of imprint cytology and histologic section. The ratio of maximal nuclear diameter of tumor cells to maximal diameter of red blood cells were 1.3-1.4:1 in nuclear grade 1, 1.6-1.7:1 in nuclear grade 2, and 2.2-2.3:1 in nuclear grade 3. These values would be guidelines for grading nuclear pleomorphism of invasive ductal carcinoma of the breast on routine surgical pathology work.