PURPOSE. To investigate diurnal variations in macular thickness and total macular volume using time domain optical coherence tomography (TD-OCT) and spectral domain OCT (SD-OCT) in healthy subjects. METHODS. Data were derived from 52 eyes of 52 healthy subjects. Macular thickness, macular volume was measured by using TD-OCT and SD-OCT at 8 AM and 6 PM. RESULTS. Comparing TD-OCT measurements, retinal thickness was significantly greater in the afternoon than in the morning in 3 of 9 grid subfields: central macular thickness (CMT) (194 vs. 190 μm; P = 0.006), superior inner area (276 vs. 273 μm; P = 0.010), and temporal inner area (261 vs. 258 μm; P = 0.006). In SD-OCT measurements, CMT, pericentral macular thickness, and macular volume did not differ significantly between the morning and the afternoon. Comparing the absolute variation in the parameters measured by the two devices, central macular thickness and temporal inner area thickness differed significantly (P = 0.015, P = 0.029), whereas other area thicknesses and macular volumes did not. Comparing the relative variation of the two devices, CMT and temporal inner area thickness differed significantly (P = 0.014, P = 0.027), whereas other area thicknesses and macular volume showed no significant difference. The moduli of variations for CMT, macular volume, and pericentral macular thickness were significantly lower using SD-OCT than using TD-OCT. CONCLUSIONS. The diurnal variation of the TD-OCT measurements was likely due to the limited repeatability of the device rather than to tissue variation. Diurnal variation was not found using SD-OCT, which has better repeatability.