Background: Neonatal sepsis (NS) is an important worldwide health problem predominantly in developing regions, Susceptibility to sepsis varies according to various neonatal factors such as innate immune response and pathogens exist in the local environment. Objective: determination of serum level and genetic Polymorphism of angiotensin converting enzyme (ACE( in NS. Methodology: this case-control study included 116 subjects, 58 in each group. The diagnosis of NS has been based on: thorough history taking, clinical signs and symptoms of NS and laboratory investigations. Blood samples were collected for inoculation on blood culture bottles followed by isolation and identification of the causative bacteria. Detection of polymorphism in ACE gene using Polymerase chain reaction (PCR) and Measurement of serum level of ACE by Enzyme linked immunsorbent assay (ELISA) were done. Results: Klebseilla was found to be the most common encountered organism.There is a significant increase in serum ACE level in NS cases as compared to healthy controls (p=0.0002). DD genotype was dominant in NS patients, while II genotype was dominant in healthy controls (p=0.00001( . The highest level of ACE was detected in DD genotypes in comparison to other genotypes with a statistically significant differences (p= 0.00005). Conclusion: ACE may play a role as a biomarker for neonatal sepsis pathogenesis.