Recently, coastal forests have received much attention because they provide both environmental and tsunami mitigation benefits. Individual trees in a coastal forest have distributed trunk diameters, so their endurance strength against tsunami fluid force will vary. In order to assess the tsunami attenuation performance of the forest, the trunk diameter distribution is essential information. This study examines the applicability of the Weibull distribution to describe the trunk diameter distributions, and discusses the time varying characteristics of the Weibull parameters as the forest grows up. Furthermore, for a model forest possessing the distributed trunk diameters, a mathematical model to quantify the tsunami attenuation rates including tree falling down effects is proposed.