Background: Post-traumatic stress disorder (PTSD) is a common and debilitating consequence of natural disaster in children and adolescents. Accumulating data show that cognitive behavioural therapy (CBT) is an effective treatment for PTSD. However, application of CBT in a large-scale disaster in a setting with limited resources, such as when the tsunami hit several Asian countries in 2004, poses a major problem. Aims: This randomized controlled trial aimed to test for the efficacy of the modified version of CBT for children and adolescents with PSTD. Method: Thirty-six children (aged 10–15 years) who had been diagnosed with PSTD 4 years after the tsunami were randomly allocated to either CBT or wait list. CBT was delivered in 3-day, 2-hour-daily, group format followed by 1-month posttreatment self-monitoring and daily homework. Results: Compared to the wait list, participants who received CBT demonstrated significantly greater improvement in symptoms of PTSD at 1-month follow-up, although no significant improvement was observed when the measures were done immediately posttreatment. Conclusions: Brief, group CBT is an effective treatment for PTSD in children and adolescents when delivered in conjunction with posttreatment self-monitoring and daily homework.