Green Infrastructure (GI) practices are widely used as source control measures in treating the stormwater. When treating stormwater, application of several GI practices in series as a treatment train has become popular over the implementation of a single treatment measure due to several advantages. However, the optimum sizing of the treatment trains has become an ongoing challenge for stormwater professionals due to various treatment measures and their different sizing combinations producing similar stormwater management benefits. Therefore, the present study proposes a novel methodology to optimize the sizing of GI treatment trains by formulating the problem as a single objective optimization. Minimization of Equivalent Annual Cost (EAC) was used as the objective function, while the target pollution reduction levels and available land area were used as constraints. Although the results of the single objective optimization should produce a single optimum result, this study has produced a set of treatment train sizing combinations which are close to the minimum cost, but with vastly different sizes of individual treatment measures in the treatment train. These sizing combinations had produced varied values for performance measures related to environmental, economic and social objectives, which made the selection of an optimum treatment train sizing combination difficult, for a particular study area. The methodology was demonstrated by using a sample treatment train, for a case study area in Melbourne, in Australia.