Introduction: Considering the Diabetes mellitus (DM) as one of the oldest condi­tions characterized in humans, it has been recognized since antiquity and considered as a major health problem at present. There is a need for efficient treatment. Aim: The aim of the present investigation was to find out whether stem cells can alleviate the well documented side effects of streptozotocin induced diabetes mellitus on the periodontal ligament of adult male albino rats. Material and Methods: Sixty adult male albino rats with body were used in this investigation. They were divided into the following three groups; Group 1: consisted of 20 animals and served as controls; Group 2: consisted of 20 animals, they were subjected to a single intraperitoneal injection of streptozotocin in a dose of 60mg. /kg body weight for induction of diabetes mellitus; Group 3: consisted of 20 animals. They were subjected to induction of diabetes mellitus in the same way as group 2 animals. One week later, the animals were subjected to a single intravenous infusion of mesenchymal bone marrow stem cells. At the end of the experiment one week later the animals were euthanized by cervical dislocation, their jaws dissected out, decalcified then processed for A. histological examination (Hematoxylin and Eosin stain) B. transmission electron microscope. Results: Histological examination in the present work showed that the treatment of diabetic rats with stem cells resulted in ame­lioration of the degenerative effects of diabetes on the periodontal ligament in group 2 and regenerative changes of stem cells in group 3 compared to the control group 1. Electron microscopic examination of the PDL showed marked regenerative changes in the density of collagen. Conclusion: Diabetic albino rats treated with stem cells revealed partial improvements, mostly in the arrangement of the fibers of periodontal ligament. The ultrastructural features showed normal fibroblasts.