In this study, we aimed to examine and compare the levels of D-Dimer, fibrinogen, CRP (C-reactive protein), and LDH (lactate dehydrogenase) in the serum of patients with type 2 diabetes with and without COVID-19. Blood samples were taken from 100 diabetic male patients with a diagnosis of COVID-19 included in the study. The mean age of the patient group with COVID-19 ( +) diabetes mellitus is 59.00 ± 12.37(min: 47, max: 71), while the mean age of the patients with diabetes mellitus in the COVID-19 (-) control group is 59.00 ± 12.37(min: 47, max: 71) 58.43 ± 13.51( min:45, max:71). In the study, mean fibrinogen, LDH, D-dimer, and CRP levels were investigated in diabetic patients with COVID-19 who did not receive any treatment (day 0) and at day 10, day 14, and day 30 after treatment. LDH, CRP, and D-dimer levels of the examined laboratory parameters were found to be higher and statistically significant in diabetic patients with a positive diagnosis of COVID-19 than in diabetic patients with a negative diagnosis of COVID-19. Higher levels of these parameters have been associated with poor prognosis in diabetic patients with a positive diagnosis of COVID-19. In addition, it was observed that fibrinogen, LDH, D-dimer, CRP, and glucose values decreased significantly over time, and the difference between each measurement was statistically significant. High fibrinogen, D-dimer, CRP, and LDH values of diabetic patients who applied to the COVID-19 outpatient clinic should be considered as biomarkers that can be used to predict the severity of the disease.