Literature study about the position of infant who has respiratory problem indicates that Prone Position (PP) is more beneficial compared to Supine Position (SP). One of monitoring action done by nurse while baby is receiving mechanical ventilator is baby’s need monitoring of concentration of inspired oxygen (FiO2). The purpose of this study is to identify the effect of Prone Position to oxygenation status (FiO2) of baby who received mechanical ventilation at NICU RSUPN dr. Cipto Mangunkusumo. The research design is pre experimental one group pretest posttest. Sample of this research is 18 babies. FiO2 level in this research was measured before PP, 30 minutes after PP, 1 hour after PP, and 2 hours after PP. Sample of this research is 18 babies by characteristics; age of 44,78±25,06 days; boys 61%; girls 39%; birth weight: 2008,33±977,84; mode of ventilator: pressure support; used synchronized intermittent mandatory ventilation and assist control; and length of use ventilator: 36,67±19,57 days. Babies’ birth weight who received mechanical ventilation and PP is 2008,33 grams on average and the standard deviation is 977,84 grams. There are no significance difference between birth weight and FiO2. The result shows that there are no significance differences of Fi02 in babies who received mechanical ventilation before and after receiving prone position. There is a significant difference in measurement before and 2 hours after PP. Moreover, there is a tendency that the more of baby’s weight, the lower of the FiO2. From that research, the recommendation of nursing implication is to improving PP intervention to babies who are in stable condition and weaning process. It is needed to conduct an advanced research with a big amount of sample, conducted by quasi experiment or true experiment method with custody variable controlling. It is also needed more analyzes for the correct PP time.