Objective: To determine the pharmacological efficacy and safety of the gentamicin regimen that adjusts intravenous dose and interval based on the gestational age (GA) in Thai neonates. Material and Method: Neonates aged ≤ 7 days, who had received gentamicin for clinically suspected or high risk of sepsis and had no contraindication to gentamicin usage were enrolled. They were stratified into four groups by GA as ≤ 29, 30-33, 34-37 and ≥ 38 weeks gestation. Gentamicin administration in each group was 5, 4.5, 4 and 4 mg/kg/dose every 48, 36, 36 and 24 hours respectively according to Neofax □ regimen. Peak serum gentamicin concentration (SGC), trough SGC and serum creatinine (Cr) were obtained. Results: Forty-nine neonates were enrolled. Forty-four (89.7%) had peak SGC within the desirable range (5-12 mg/L). Three neonates had slightly high peak SGC. Their peak SGCs were 13.0, 12.21 and 12.20 mg/L. Two neonates had slightly low peak SGC. Their peak SGCs were 4.91 and 4.4 mg/L. All neonates had trough SGC below 2 mg/L. None had significant rising of serum Cr during the present study period. Conclusion: This gentamicin regimen yielded good pharmacological efficacy and safety in Thai neonates, who were in the first week of life and had no renal function impairment.