Purpose: Assess the effect of diabetes mellitus on ganglion cell complex (GCC) thickness in diabetic eyes without retinopathy compared to control non diabetic eyes. Assess the correlation between GCC thickness and other findings: HbA1C, type and duration of diabetes, central foveal thickness (CFT), cup/disc ratio, intraocular pressure (IOP), refraction, and best corrected visual acuity (BCVA). Methods: Two groups of eyes were included; diabetic eyes without retinopathy (46 eyes) and control non diabetic (21 eyes). Exclusion criteria included other ocular pathologies, previous ocular surgeries or trauma. All subjects underwent medical and full ophthalmological history, slit lamp examination, IOP, dilated fundus examination and measuring GCC thickness and CFT using the RTVue® SD-OCT (Optovue, Inc.) at Cairo University Hospital. Results: The GCC focal loss volume (FLV%) was significantly more in diabetic eyes than normal eyes. The GCC global loss volume (GLV%) was significantly more in type I DM than type II DM. Average GCC, superior GCC and inferior GCC thickness were significantly less in type I DM than type II DM. CFT was positively correlated to the average, superior and inferior GCC thickness and negatively correlated to the GLV% and FLV% in diabetic eyes. C/D ratio in diabetic eyes was negatively correlated to the average, superior and inferior GCC thickness and positively correlated to the GLV%. The FLV% was negatively correlated to the refraction, BCVA, level of HbA1C in diabetic group. Conclusions: Significant GCC thinning in diabetes predates retinal vasculopathy, which is mainly focal rather than diffuse. GCC loss is more with type I DM than type II DM. It has no preference to either the superior or inferior halves of the studied macular region. Increase of myopic error is significantly accompanied with increased focal GCC loss. GCC loss is accompanied with increased C/D ratio in diabetic eyes.