AIM: To investigate the intraocular retinal thickness asymmetry of peripapillary retinal nerve fiber layer (pRNFL) and macular layers measured by spectral-domain optical coherence tomography (SD-OCT) in patients with early stage of primary open angle glaucoma (POAG) and normal tension glaucoma (NTG). METHODS: A total of 117 patients with early stage of glaucoma (54 patients with POAG and 63 patients with NTG) and 32 normal subjects were recruited for the study. The pRNFL thickness, total macular layer (TML) thickness, and isolated inner macular layer (IML) thickness were measured by SD-OCT. Hemisphere TML thickness asymmetry measured by the posterior pole asymmetry scan was evaluated. Thickness differences of pRNFL and IML between superior and inferior quadrants were calculated. Asymmetry indices (AIs) of the pRNFL, TML and isolated IML were also computed. Areas under the receiver-operating characteristic curves (AROCs) were generated to determine the diagnostic capabilities of different parameters. RESULTS: Intraocular pRNFL thickness differences and AIs between the superior and inferior quadrants were significantly different between normal and NTG groups (P=0.009 and P<0.001, respectively). Intraocular pRNFL thickness differences and AIs between the temporal-superior and temporal-inferior sectors were also significantly different between normal and NTG groups (P=0.035 and P<0.001, respectively). The thickness differences and AIs of TML between superior and inferior hemispheres were significantly different between normal and NTG groups (P=0.001 and P=0.001, respectively) and between normal and POAG groups (P=0.032 and P=0.020, respectively). The thickness differences and AIs of macular ganglion cell layer (mGCL) between superior and inferior quadrants were significantly different between normal and NTG groups (P=0.013 and P=0.004, respectively), and between NTG and POAG groups (P=0.015 and P=0.012, respectively). The thickness difference of TML between superior and inferior hemispheres showed the highest diagnostic capability for early NTG eyes (AROC=0.832). CONCLUSION: Intraocular retinal thickness asymmetry in pRNFL, TML and mGCL are found in early stage of NTG. Hemisphere TML thickness asymmetry is also found in POAG eyes. Asymmetry analysis of retinal thickness can be an adjunctive modality for early detection of glaucoma.