Pirpose: The aim of this study to analyze the association between history of diabetes mellitus (DM) with risk of prostate cancer (PCa) and cancer grade among men undergoing radical prostatectomy for PCa. Materials and Methods: 50 patients with DM and 50 patients without DM who undervent radical prostatectomy (RP) were included in the study. Age at biopsy, height, weight, digital rectal examination (DRE), pre-biopsy PSA levels, prostate volume, histopathologic diagnosis after surgery and gleason scores were collected data from all patients. Histologic material obtained at biopsy was given a Gleason score; tumours with a Gleason score ${geq}7$ were considered high grade and <7 were considered low grade. Results: The mean age at the time of biopsy was 63.7 in patients with DM and 61.6 in patients without DM. Diabetic men had significantly lower PSA levels (p=0.01). Mean PSA level $7.04{pm}2.85$ in patients with DM and $8.7{pm}2.86$ in patients without DM, respectively. Also, diabetic men had higher RP tumor grade than men without DM (p=0.04). We found that HbA1c levels were higher in patients who have high grade prostate cancer (p<0.05). Conclusions: Diabetic men undergoing RP have lower PSA levels and have significantly higher grade PCa. We must be careful for screening PCa in patients with DM. Although the patients had lower PSA levels, they might have high grade disease.