This study was conducted to quantify of micronucleus frequencies in human umbilical cord blood by supravital staining method with acridine orange, and to find some factors that affected on micronucleus frequncies in humans. In this study, we used umbilical cord blood of new born infants that have sufficient reticulocytes compared with adult peripheral blood. The cord bloods were taken after childbirth from 60 normal infants in industrial and coastal region in Korea. The total of 3 ${mu}ell$ cord blood was applied to slide coated with acridine orange, and micronuclei were observed under fluorescent microscopy. Demographic factors and independent variables were collected from mothers by questionnaire. The frequencies of micronuclei in umbilical cord blood of new born infants were 0-5 per 2,000 reticulocytes by supravital staining method, and mean value and standard deviation were 1.75$pm$0.97. There were no significant difference by the regions, smoking habits of father or mother. However, age of mother showed significant positive correlation with frequencies of micronuclei (p<0.05). Smoking at home by fathers also was found as a significant variable by muliple regression analysis. Therefore, further studies would be needed for genotoxicological evaluation of new born infants by microneuli test using supravital staining method.