Aims: The aim of this study was to evaluate the expression of Ki-67(LI) in breast cancer lesions among Sudanese females. Study design: Cross-sectional descriptive study. Place and Duration of Study: Radiation and isotope center at Khartoum (RICK), Sudan, during the period between October 2012 and September 2013. Methodology: Tissue sections obtained from 200 formalin-fixed paraffin-embedded tissue blocks of breast cancer lesions were immunohistochemically stained using monoclonal antibody (MIBI) for Ki-67. Results: The study showed significant differences in expression of Ki-67 in different grades of triple positive and triple negative breast cancer lesions. Conclusion: Expression of Ki-67 in breast cancer is an important prognostic factor as it significantly associated with aggressive tumors.