Objective: Schizophrenia(SCZ) is a heritable severe mental disorder with unknown pathogenic mechanisms. It is known that oxidative stress(OS) plays a crucial role in the pathophysiology of SCZ. Our study aimed to evaluate whether there is a relationship between the severity of illness and OS biomarkers in SCZ patients and first-degree-relatives of patients(FDRs). Methods: This study included 50 adult patients with SCZ, 50 unaffected FDRs, and 50 controls. OS biomarkers, myeloperoxidase(MPO), total oxidant status(TOS), total antioxidant status(TAS), total thiol(TT), and native thiol(NT) levels were measured by different photometric methods. Disulfide(DS) and oxidative stress index(OSI) were calculated with mathematical formulas. Results: TOS, DS, and OSI levels were significantly higher, and TAS, TT, and NT levels were lower in SCZ and FDRs than controls. In SCZ group, MPO activity was higher compared to relatives and controls. Results in this study did not provide a strong correlation between Positive&Negative Syndrome Scale(PANSS) and other biomarkers. There was just a low negative correlation between TT and PANSS in SCZ group(p=0,041,r=-0,297). Conclusion: OS increased significantly in SCZ, and FDRs increased slightly than controls, indicating the presence of OS in the etiology of the disease. High OS may be an effective factor for SCZ in later years.