Acute respiratory illness is an important cause of morbidity and mortality in children. Bronchiolitis, Pneumonia and Acute respiratory distress syndrome are some of the important disorders for which children require respiratory support. 30 to 64% of all children admitted in paediatric intensive care unit require ventilator support. Aims: To do a prospective observational study, comparing the relationship between Pao2/Fio2 (PF) ratio and Spo2 /Fio2 (SF) ratio in critically ill children requiring respiratory support in a Paediatric ICU of a tertiary care center in North India. Materials and Methods: The critically ill children who required respiratory support in the form of mechanical ventilation , attending to our pediatric ICU in last 6 months are taken in our study. Total 125 patients included in this study. Result: In our study, we also compared SF ratio and oxygenation index more than four and found a good correlation with an AUC of 0.739.The recent PARDS guidelines have given Oxygen saturation Index (OSI) cut off more than 5 to diagnose ARDS. When we compared oxygenation index more than 4 with oxygen saturation index, we found a good AUC of 0.897 in the ROC with 91% sensitivity and 73% specificity to diagnose ARDS. Thus, we validated this guideline in our study. Conclusion: Our study shows that SF ratio can be used as a reliable, non-invasive, surrogate marker for PF ratio to diagnose ARDS and also as a screening tool to assess the severity of respiratory illness in resource limited settings where arterial blood gas is not available