The aim of this clinical study was to compare the effect of cavity size on the success rate of occluso-proximal restorations made with Atraumatic Restorative Treatment (ART) approach.The study was conducted at the faculty of oral and dental medicine, Cairo University, Egypt. ART restorations were placed in 52 permanent molars of 52 young adults of both genders with mean age 15 years. Half of the patients had small size occluso-proximal carious lesions (si/sta 2.2) and the other half had large size carious lesions (si/sta 2.3). Teeth of both groups were treated using the ART approach and all the cavities were restored with Fuji IX GP HVGIRM. ART restorations were evaluated after 6 months using ART criteria adopted for proximal restorations. The results showed cumulative survival rate of ART occluso-proximal restorations was 100% and 96.6% respective values for the success rate of small and large restorations at six months evaluation period with no statistical significant difference .At the same time no statistical significant difference was recorded for both genders whether the cavity size was small or large. Conclusion: Atraumatic restorative treatment (ART) was successful as interim treatment up to six months for occluso-proximal restorations in permanent teeth regardless to the cavity size.