Pediatric Ulcerative Colitis (UC) patients with moderate to severe disease activity at high risk of colectomy. Early use of biologic agents will likely be more effective. But there were no studies identified that compared a strategy of upfront biologic-based therapy versus gradual step-up therapy. In our study, newly diagnosed moderate to severe pediatric UC patients (6-18 years old) will be randomly divided into infliximab (IFX) treatment group (Top down group, TD) and corticosteroids (CS) treatment group (Step-up group, SU). Mucosal healing rate at week 12 will be compared between the two groups. The relapse rates and sustained durations of remission within one year will also be evaluated.