Breakwaters are very important to keep harbors calm. However, they have been severely damaged quite often since they are directly exposed to large waves from open sea. Port and Harbor Research Institute of Ministry of Transport edited four volumes of damged breakwaters, and published them as technical reports. The present paper investigates the characteristics of damaged breakwater referring to the technical reports and statistically derives main factors closely related to the damages. It was founded that aspect ratio of a casson, and horizonal wave force ratio to weight of the casson show close relationship with breakwatars damage modes