Abstract: Nuclear morphometry of the slight hyperplastic epithelim,the active hyperplastic epithelium,the atypical hyperplastic epithelium and the normal epithelium of the mammary gland in rats were observed by image analysis system. 4 morphometric parameters(area, perimeter,longest diameter and shape factor)of the cell nuclei were determined.The results showed that there are significant or great significant difference among the active hyperplasia,the atypical hyperplasia and the normal mammary epithelium (P＜0.05, P＜0.01), and no significant difference between the slight hyperplasia and the normal mammary gland (P＞0.05).Thus,we suggest that with increasing of the hyperplastic degree of mammary epithelium,the nuclei volume will enlarge,the nuclei shape will variate,and the risk of breast cancer will increase.