Purpose:.Glaucoma is an eye disease that can lead to irre-versible optic nerve damage and cause blindness. Optical co-herence tomography.(OCT) allows an early diagnosis of glau-coma by the measurements of the retinal nerve fiber and optic disc parameters.A retrospective study was designed to analyze the effects of the measurement of the retinal nerve fiber layer (RNFL) thickness and the optic disc tomography by spectral-domain OCT on the early diagnosis of suspected glaucoma and primary open angle glaucoma (POAG). Methods:.This was a clinical case-control study. The RNFL thickness around the optic disc and optic disk tomographic pa-rameters of the control (n=51, 98 eyes), suspected glaucoma (n=81,.146 eyes), and POAG groups (n=55, 106 eyes) were measured by OCT. The parameters included superior, inferi-or, nasal and temporal mean RNFL thickness,.disc area (DA), cup area (CA), rim area (RA), disc volume (DV), cup volume (CV), rim volume (RV), cup/disc area ratio (CA/DA), rim/disc area ratio (RA/DA), cup/disc volume ratio (CV/DV) and rim/disc volume ratio (RV/DV). Results: Superior, nasal, and mean RNFL parameters, DA, CA,RA, DV, CV, CA/DA, RA/DA, CV/DV and RV/DV significantly differed among three groups by single-factorial ANOVA. Inferior and temporal RNFL thickness significantly differed between the control and POAG groups. No significant difference was observed in RV among three groups. In the POAG group,.the maximum area under the ROC curve (AROC) of mean RNFL thickness was 0.845. The maximum AROC of optic disk parameters was RA/DA (0.998), followed by CA/DA (0.997). The AROC of CA, RA, CV, and DV were all>0.900. Conclusion:.OCT may serve as a useful diagnostic modality in distinguishing suspected glaucoma from POAG.