Abstract: This study was aimed to recognize factors effective on weaning from mechanical ventilation and determine the reasons of unsuccessful Spontaneous Breathing Trial (SBT) and reintubation. The study population consisted of 202 critically ill pediatric patients who received mechanical. When the patient was enrolled in the study, mechanical ventilation support was stopped and the patient underwent a 2 h trial of Spontaneous Breathing Trial (SBT), at the end of the trial if PaCO2 was in normal range was extubated. Of the 202 patients who underwear SBT, 141 attempts had successful trial. The remaining 61 patients had sign of poor tolerance during the trial of spontaneous breathing and were reconnected to the ventilator. One hundred and forty one patients (69.8%) successfully passed the trial. 17 (12%) of above mentioned group required reintubation within 48-72 h. The overall success and failure rate was 61.3 and 38.7%, respectively. Mortality rate in patients who did not tolerate SBT or were extubated, or required reintubation were 13% (8 patients), 11.3% (14 patients) and 23.5% (4 patients), respectively. The most common reasons for reintubation were neuromuscular disease (58.8%) congenital cardiac disease (23.5%) and aspirative pneumonia (17.6%). The finding indicates that two third of intubated patients, respond successfully to SBT and could be extubated and the neuromuscular diseases is the main cause of reintubation.