The extraction mechanism of metal ions in wide concentration ranges of extractant is investigated. Not only in the low extractant concentration range but also in the high concentration range, the extraction rate is affected by aqueous conditions such as pH and metal concentration. Furthermore, the rate is increased with an increase in the extractant concentration below about 2500 mol/m(3) and adversely decreased over similar to 2500 mol/m(3). It is suggested that, in a high extractant concentration range, diffusion resistance of the extracted complex through liquid organic laminar film strongly affects the overall extraction rate. The extraction mechanism is evaluated using a modified interfacial reaction model considering an increase in the viscosity of the organic phase with an increase in extractant concentration by applying the Wilke-Chang equation and Lima's equation. As the result, both extraction mechanisms in the ranges of low and high extractant concentrations are the same in each other, which suggests that the proposed model is reasonable.