Intramammary sentinel lymph node excision during breast conservative surgery was performed, in this case report, using a prototype intraoperative gamma probe. In contrast to the four axillary sentinel lymph nodes that were subnormal, the excised intramammary sentinel lymph node was massively invaded by cancer cells. Therefore this finding had profound implication for the staging of the tumor and for treatment selection. This case report illustrates that an efficient intraoperative gamma probe is useful to locate and remove intramammary sentinel lymph node in breast cancer patients treated with breast conservation.