There are many types of coastal vegetation. However, the tsunami mitigation effects in relation to the tree density distribution in a forest have not been investigated yet. The objective of this study is to experimentally investigate the tsunami mitigation effects by the combination of vegetation with different tree density under the same vegetation thickness (dn). Vegetation is modeled by circular cylinders and arranged in a wave channel with slope. In the experiments, water level, velocity, force and run-up height are measured. When tree density is dense, the run-up height and maximum velocity behind vegetation become smaller than the sparse case. This implies that the coastal vegetation for tsunami mitigation should be as dense as possible, even if the vegetation thickness is the same.