Background: Vitamin D plays a major role in growth and development of children. Optimal levels of vitamin D in first year of life are found to be preventive for lower respiratory tract infections. Aim of Study: To assess the effect of vitamin D supplementation on treatment of bronchiolitis. Patients and methods: This study is prospective follow up study conducted in Pediatric Wards of Al-Imamain Medical city in Baghdad through the period from 1st of January, 2018 to 31st of January, 2019 on convenient sample of 200 children with bronchiolitis divided into two group .group A include 98 children who were managed by traditional treatment of bronchiolitis (oxygen ,fluid ,frequent sucking of secretion ) and V.D supplementation (400 iu)daily and group B that include 102 children managed with traditional treatment only . The vital signs and hospital stay duration of children were assessed daily . Results: No significant differences were observed between two study groups regarding demographic, social, clinical and feeding characteristics. Also no significant differences were observed regarding respiratory rate at admission and at discharge. There was a significant difference regarding heart rate at discharge (p=0.05). A significant association was observed between children with vitamin D and shorter hospital stay duration (p<0.001). Conclusions: Vitamin D supplementation is important in treatment of bronchiolitis.