PurposeTo evaluate the clinical performance of two nano-hybrid giomer restorative composites; Beautifil II and Beautifil Flow Plus F00 with FL-Bond II adhesive system in class I posterior restorations during three-year period. Materials and methodsTwenty patients joined this study with age ranging from 20 to 35 years. Each patient has to present two permanent upper or lower molars of the same side requiring new class I restorations of primary carious lesions to be restored by both tested materials. Two clinicians examined the twenty patients with 40 restorations (20 for each restorative material) clinically using Modified USPHS/Cvar & Ryge Criteria for direct restoration for a period of three years with an examination interval 6 months. ResultsData was collected and statistically analyzed using SPSS version 18. Friedman's test showed no significant changes to all modified USPHS criteria for each material during the three-year evaluation period. Fisher's exact test showed no significant changes between materials in postoperative sensitivity, recurrence of caries or retention of restoration. The significant changes recorded were after three years period follow up between the two materials; Beautifil flow plus F00 has significantly better marginal adaptation (P < 0.01), marginal discoloration (P = 0.01), surface roughness (P = 0.01) and surface morphology (P < 0.01) versus Beautifil II. ConclusionBeautifil Flow Plus F00 (zero flow) restorative material achieved clinically better significant acceptable results than Beautiful II after three years of service in conservative class I cavities.