The purpose of the current investigation was to improve the solubility of daidzein, a poorly water-soluble drug which exhibits low oral absorption bioavailability, in a self-micro-emulsifying drug delivery system that is suitable for oral administration. A carefully executed central composite design was applied to screen the optimal formulation of daidzein SMEDDS. The formulations were characterized by solubility of the drug in the vehicle, droplet size, and emulsification time. The optimal formulation consists of 20 % ethyl oleate, 64 % OP emulsifier, and 16 % polyethylene glycol 400. The droplet sizes of drug-free SMEDDS and drug-loaded SMEDDS were 66.4 nm and 77.9 nm respectively. Additionally, the dissolution rate of daidzein from SMEDDS was significantly enhanced in comparison to pure drug. The data suggested that the daidzein SMEDDS was prepared successfully.