Abstract : This randomized controlled clinical trial is aimed to evaluate the clinical performance of high-viscosity bulk-fill composite in the restoration of second mandibular primary molars compared to conventional incremental composite. The studied sample consisted of 60 second mandibular primary molars in 59 children aged between 5-8 years, these cases were allocated randomly into four groups: Group 1 (experimental, n=20): High Viscosity Bulk Fill Composite with total etching (HVBFC-T), Group 2 (experimental, n=20): High Viscosity Bulk Fill Composite with Self-etching (HVBFC-S), Group 3 (Control n=20): Conventional Incremental Composite with Total etching (CIC-T). Post-treatment clinical follow-ups were conducted during 3, 6, and 12 months using The Modified United States Public Health Service (USPHS) criteria. Data were analyzed using Kruskal- Wallis test. The clinical success rate reached 100%, 95%, and 95% in HVBFC-T, HVBFC-S, and CIC-T groups respectively. Statistical analysis revealed no statistically significant differences among HVBFC-T, HVBFC-S, and CIC-T groups in overall clinical evaluation (P-value= 0.375). Bulk fill composite with two dentine bonding systems (Total Etch, Self-Etch) showed satisfactory clinical performance in primary molars during the 12-month follow-up period.