Recently, self-assembly synthesis of metal nano-particles has attracted great interest due to its unique size dependent properties. In this study, an easy synthesis would be developed to form Au nano-particles which are without toxicity. The Au particles could be applied in biological and biological toxicity experiments. Some phenomena could be discovered in this paper. First, the concentration ratio of Au and trisodium citrate would be adjusted to control the Au nano-particles size. And the particle size is limited because of the size dependent energy would be discovered in this paper. The minimum particle size is also found. The one phase liquid reduction method is easy and without toxicity, it is low-cost, high-throughput, and suitable to be applied in biological and biological toxicity studies.