Background: Diagnostic criteria for acute lung injury(ALI) and ARDS requiring acute onset of disease, chest radiograph demonstrating bilateral pulmonary infiltrates, lack of significant left ventricular dysfunction and 2 2 Fio Pao (PF) ratio ≤ 300 for ALI or ≤ 200 for ARDS. recent criteria is requiring invasive arterial sampling. The pulse oximetric saturation Spo2/Fio2 (SF) ratio may be a reliable non invasive alternative to the PF ratio. Methods : In this cross sectional study , Enrolled 70 patient ALI or ARDS that Admitted in Tabriz children’s hospital PICU. Included in the analysis were corresponding measurement of Spo2,Fio2, Pao2,charted within 5 min of each other And computed SF and PF to determine the relationship between SF and PF ratio. SF threshold values were determined to replacement PF ratio for diagnose ARDS and ALI. Results: The relationship between SF and PF ratio was described by the following regression equation SF=57+0/61PF (P<0/001). SF ratios of 181 and 235 corresponded of PF ratio 300 and 200. The ALI SF cutoff of 235 had 57% sensitivity and 100% specificity, and ARDS, SF cutoff of 181 had 71% sensitivity and 82% specificity. Conclusion: SF ratio is a reliable noninvasive marker for PF ratio to identify children with ALI or ARDS and can be replaced pulse oximetry by arterial blood sampling.