It is known that cardiovascular complications plays important roles in the development of diabetes mellitus (DM) and platelet dysfunction is one of the key reasons which led to microangiopathy. This study was designed to investigate the mitochondria function changes of platelet in DM rats and DM patients. The results showed that the platelets viability, platelet adenosine triphosphate (ATP) content and platelet mitochondrial ATP content of DM rats were lower than that of normal rats; when incubated in vitro for 24 h, platelet number and mitochondrial membrane potential (MMP) of DM rats were lower than that of normal rats, reactive oxygen species (ROS) was higher than that of normal rats. For DM patients, their platelet number and ROS were higher and MMP was lower than those of normal people; when incubated in vitro for 24 h, platelet viability of DM patients was lower than that of normal people. Platelet ultra-microstructures of DM rats and DM patients were abnormal. These results suggested that platelet mitochondrial function of both DM rats and DM patients was impaired when compared to normal rats and normal people, respectively. Platelets may be applied as a biomarker to observe the mitochondrial changes during DM.