Introduction: We aimed to evaluate plasma thiol and disulphide levels as an additional marker to prostate specific antigen (PSA) in the diagnosis of prostate cancer (PCa). Methods: Between March 2017 and January 2018 prospective study was conducted among patients with PSA levels of 2.5-20 ng/mL without suspicion of malignancy in rectal examination and who underwent prostate needle biopsy. Patients were divided into two groups according to PSA level as 2.5-10 ng/mL (Group 1) and 10.01-20 ng/mL (Group 2). Diagnostic efficacy of thiol, disulphide and PSA levels were measured by ROC analysis. Results: A total of 76 patients were included in the study. There were 49 patients in group 1 and 27 patients in group 2. There was no significant difference between two groups in terms of PSA density and prostate size. In Group 1, area under curve (AUC) was higher in PSA than other parameters with statistically significant difference (p<.05). In group 2, AUC of native and total thiol was higher than PSA but there was no statistically significant difference for AUC in parameters. Conclusions: We think that plasma thiol test may be used in diagnosis of prostate cancer while PSA levels between 10 to 20 ng/mL. However, further studies are required.