Diabetes mellitus is one of the most prevalent metabolic disorders. Carbon nanotubes have the advantage to cross the plasma membrane without damaging the cells, improving the biological effect of a drug and reducing its side effects. In the present study, the effect of metformin and metformin-conjugated nanotubes was investigated on blood glucose level in the streptozotocin-induced male diabetic rats. Diabetes in the animals was induced with a single dose of streptozotocin (60 mg/kg; i.p.) and after 3 days the blood glucose was analyzed. Animals showing fasting blood glucose higher than 250 mg/dL were considered as diabetic rats. The animals were treated with metformin and metformin-conjugated nanotubes (150 mg/kg; p.o.) daily and every 48-h for 1 week. Changes in animals' serum blood glucose level were evaluated daily during the treatment period. The results of this study showed that metformin reduced blood glucose levels in diabetic animals. Metformin-conjugated nanotubes significantly reduced the blood glucose levels in diabetic rats (p < 0.01). There was no significant difference in blood glucose level between metformin and metformin-conjugated nanotubes groups (p > 0.05). However, when both formulations of metformin were administered every 48-h, metformin-conjugated nanotubes reduced glycaemia for a longer time than metformin alone (p < 0.001). This study showed that the metformin-conjugated nanotubes would be able to reduce the blood glucose, prolong drug delivery and efficacy duration in animals which were treated with metformin-conjugated nanotubes compared with metformin alone.