Background: Chorioamnionitis is the association of microorganisms infection in fetal membrane, placental membrane and amniotic fluid. About 2 to 4% of chorioamnionitis occurs in full-term deliveries, but higher than this prevalence in preterm birth. Objectives: The objective of this study was to detect the most dominant bacteria of chorioamnionitis and evaluate correlation of the presence of microorganisms infection in placental membrane, fetal membrane and amniotic fluids in women with preterm birth. Methods: It was cross section study where 20 women with preterm premature rupture of membrane, premature rupture of membrane or preterm birth. By caesarean section or without caesarean section with a group of control of 10 women with term delivery. After delivery immediately the samples swab of amniotic fluid, fetal membrane and placenta membrane sample were placed in separate sterile container (swabs Stuart plastic) in the hospital, where it was stored at temperatures ranging between 8oC-20oC. Culture technique, gram staining and biochemical test were used to identify the microorganisms in this study at INES-Ruhengeri Microbiology laboratory. At the Ruhengeri Referral Hospital and analyzed according to INES Microbiology Laboratory standard operating procedures. Results: in 20 women with preterm birth, the most dominant mircoroganism was yeast 28.4% and mould 28.4%, other infections were caused by Escherichia coli, with 9.3%, Klebsiella species with 3.7%, Streptococcus species with 9.3%, Staphylococcus species with 9.3%, Candida albican with 11.7%. Those microorganisms show the association in fetal membrane, placenta membrane and amniotic fluid, of all women suspected to have chorioamnionitis. In 10 samples of control group, for women with term birth, same microorganisms were found like in fetal membrane were (Staphylococcus species 11%, mould 33%, and Yeast 56%), in placenta membrane were (Staphylococcus species 20%, mould 30%, Yeast 50%) and Amniotic fluid were (Staphylococcus species 0%, mould 33%, Yeast 67%). Conclusion: Chorioamnionitis can be in women with preterm birth or with term birth, untreated microorganism infections will cause a big problem of chorioamnionitis in pregnant women this will conduct to preterm morbidity and mortality.