During the thousands years’ development of Chinese traditional timber building, a large timber structure construction methodology system based on experiences has been constructed. This project chose “Rafter” element from traditional timber structure system, trying to reproduce and develop this structure element through structure simulation & optimization software and robotic fabrication method. The overhanging rate of rafter has been clearly ruled inYin Zao Fa Shi and we hope to optimize it by Millipede and Galapagos. Based on the rate we got from optimization and a kind of basic self-supporting triangular component consisted of three wood sticks, we designed a new structure system. The structure installation is composed of seven components generated from the same logic but varied from each other in size, joins and angles. The exactness of robotic fabrication can solve these problems perfectly, so that the structure purpose of this installation can be achieved excellently.