The fragility function of mangrove forest was proposed based on field surveys and the numerical analysis focusing mangrove forests affected by the 2004 Indian Ocean tsunami. The curve of growth for mangroves (Rhizophora spec.) was also proposed in order to estimate the change of the tsunami reduction effect depending on the age of them. Using these proposed functions, the numerical modeling was carried out to evaluate a tsunami reduction effect. As the results, we found that the mangrove forest with 10-year-old is mostly destroyed and the reduction effect rapidly decreases when the tsunami inundation depth exceeds 4m.