Aim of the study: This study aimed to evaluate the clinical performance of 2 highly filled flowable resin composites in class I cavities over a period of 18 month follow -up. Materials and Methods: a total of 48 moderate-sized class I carious lesions were selected in 24 patients aged between 20-45 years. Class I cavities were divided comprising 2 equal groups (n = 24). Group I: Prime &bond universal adhesive was applied to the cavity walls, followed by bulk fill application of SDR flow+, while in Group II: G-Premio Bond universal adhesive was applied, followed by incremental application of G-aenial universal injectable composite. All restorations were clinically evaluated at baseline (24 h), 6, 12, and 18 months using modified (USPHS) criteria. Marginal adaptation was further objectively examined during all the evaluation periods by the inverse replica technique which were observed under (ESEM). Results: st the 18-month follow-up, 48 restorations were evaluated in 24 patients. After 18 months, the difference between both highly filled flowable composite restorations was not statistically significant with respect to all evaluation parameters (p < 0.05). No secondary caries was observed. Conclusions: both tested materials with their application techniques showed acceptable comparable clinical effectiveness over 18 months follow up