The role of exclusive enteral nutrition (EEN) in the induction of remission in pediatric patients with Crohn's disease had been well documented. But the role of EEN in children with Ulcerative Colitis (UC) was not clear. In this study, EEN will be combined with corticosteroids or infliximab in the induction of remission in pediatric UC patients with moderate to severe disease activity. The mucosal healing rate at week12 will be compared between the two groups (combine with EEN group VS non-combine group).