History and Objectives: In order to histological evaluation of os cordis, this study was performed on some of its specimens at the ward of anatomy of faculties of medicine and dentistry of Shaheed Beheshti University of Medical Sciences in 1377.

Materials and Methods: This study was performed by descriptive method. Five samples of os cordis were obtained from hearts of sheep and were fixed by formalin. After decalcification by formic acid and dehydration, it was cleared and embedded in paraphine. Then these sections, with 5 microns thickness, were stained by the hematoxylin and eosin method.

Results: In slides of each 5 samples of the os cordis hyaline cartilage and spongeous bone were observed. The central spaces of the bone contained bone marrow with hematopoietic and megakaryocytic cells.

Conclusion: These os cordis showed hematopoietic activity so further investigation on the vascular communications and their function is recommended.