Background: Asthma is the most common chronic disease in childhood and cause of hospitalization.Intravenous magnesium sulfate is recommended as a complementary therapy for asthma exacerbation.This study investigate the effect of oral magnesium sulfate on moderate and severe asthma exacerbation in children.
 Methods: This double-blind clinical trial was performed on 40children 5-12 years old with asthma exacerbation who referred to Pediatric Emergency Department of Quaem Hospital in Mashhad, Iran. Patients were randomly assigned into two groups , received magnesium sulfate (300 mg every 12 hours) or placebo capsules for 5 days. Asthma severity score based on Siriraj clinical asthma score (SCAS) contained in the questionnaire was recorded for each patient at 0, 2, 6, and 24 hours after first visit, and then daily up to 5 days. The patients were divided into three groups by researcher: A(Mild) :0-8, B(Modrate):8-16, C(Severe):16-24.
 Results: The mean age of subjects was 8.20 ± 2.49 age and mean weight was 26.32 ± 8.65 kg. Classification of asthma severity in the magnesium group included 5 patients in A, 14 in B and 1 in C and in placebo group 8 in A,8 in B and 4 in C .The present study, controlling the effect of weight as a covariate, didn't indicate a significant effect of oral magnesium sulfate on asthma symptoms in 5-12 years old children (P=0.15); however, after considering the severity variable as moderator, indicated a minor effect on symptom improvements in B score.
 Conclusion: Adminstration of oral magnesium sulfate Within 5 days, didn't improve asthma exacerbation in5-12 years old children.So, more studies with different dose and duration are needed to determine efficacy of oral mgso4 in asthma exacerbation.